Method for forming multilayer coating film

ABSTRACT

The present invention provides a method for forming a multilayer coating film including an intermediate coating film formation step of applying an aqueous intermediate coating composition to an object to be coated to form an uncured intermediate coating film; a base coating film formation step of applying an aqueous base coating composition onto the resulting uncured intermediate coating film to form an uncured base coating film; and a curing step of curing the resulting uncured intermediate coating film and the base coating film by heating.

TECHNICAL FIELD

The present invention relates to a method for forming a multilayercoating film using an aqueous intermediate coating composition and anaqueous base coating composition.

BACKGROUND OF THE INVENTION

On the surface of an object to be coated such as an automobile body, aplurality of coating films having various roles are formed sequentially,and thus the object to be coated is protected, as well as beautifulappearance and an excellent design. A common method for forming such aplurality of coating films, e.g., for a steel plate, is a method inwhich an undercoating film such as an electrodeposition coating film isfamed on an object to be coated that is excellent in conductivity, andthen an intermediate coating film, a base coating film, and a clearcoating film are formed thereon one after another.

In view of further request for reducing burden on the environment suchas energy saving and reduction in CO₂ emission, it is required to lowera heat curing temperature in a coating film formation. Moreover, inautomobile manufacturing fields, further reduction in weight of anautomobile body is required according to the development of electricvehicles. The reduction in weight of an automobile body brings about animprovement in fuel economy, and therefore, it is also effective interms of energy saving and reduction in CO₂ emission. One way to reducethe weight of an automobile body is to replace a steel plate part with aresin part.

In conventional coating for steel plates and resin members, it is commonthat different coating compositions are used respectively inconsideration of the characteristics and softening temperature of eachmember. On the other hand, in coating of automobile bodies, for thepurposes of simplifying coating step and coating management andimproving hue consistency in a coated product, it has been demanded tocommonize coating compositions to be used for coating variouscomponents. In the case of commonizing the coating composition for asteel plate and that for a resin member, however, it is necessary toadjust the curing temperature of the coating composition to atemperature lower than the conventional curing temperature inconsideration of the heat resistance of the resin member. Moreover, inthe case of forming a coating film on an object to be coated having botha steel plate part and a resin part, deformation may occur during heatcuring due to a difference in thermal expansion coefficient between themembers. Therefore, it is extremely important to lower the heat curingtemperature and minimize the influence of the thermal history on eachmember in commonizing the coating composition.

On the other hand, lowering the heat curing temperature may reduce thecrosslinking density of a resulting coating film and coating filmperformance such as water resistance and chipping resistance maydeteriorate.

JP-A-2011-131135 (Patent Document 1) discloses a method for forming amultilayer coating film that involves applying an aqueous intermediatecoating composition to a substrate having both a steel plate and aplastic substrate to form an intermediate coating film, then applying anaqueous base coating composition to the formed intermediate coating filmto form a base coating film, then applying an organic solvent type clearcoating composition to foul a clear coating film, and heating and curingthe three layers, namely, the intermediate coating film, the basecoating film, and the clear coating film, in which the aqueous basecoating composition contains (a) an acrylic resin emulsion, (b) awater-soluble acrylic resin, (c) a melamine resin, and (d) a propyleneglycol monoalkyl ether. A multilayer coating film obtained by thisforming method may not exhibit sufficient chipping resistance when beingcured, for example, at a temperature of 100° C. or less.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: JP-A-2011-131135

SUMMARY OF INVENTION Problems to be Resolved by the Invention

The present invention solves the above-mentioned conventional problemsand an object thereof is to provide a method for forming a multilayercoating film by which a multilayer coating film having excellent coatingfilm performance can be formed even by low temperature curing.

Means of Solving the Problems

The present invention provides the following aspects to solve theaforementioned problems.

[1]

-   A method for forming a multilayer coating film, wherein the method    comprises:

an intermediate coating film formation step of applying an aqueousintermediate coating composition to an object to be coated to form anuncured intermediate coating film;

a base coating film formation step of applying an aqueous base coatingcomposition onto a resulting uncured intermediate coating film to forman uncured base coating film; and

a curing step of curing the resulting uncured intermediate coating filmand the base coating film by heating, wherein

the aqueous intermediate coating composition is an aqueous intermediatecoating composition comprising:

-   -   an aqueous resin having a hydroxyl group and a carboxyl group        (A1);    -   a polyisocyanate compound (B); and    -   a hydrophilicized-modified carbodiimide compound (C),

the aqueous base coating composition is an aqueous base coatingcomposition comprising:

-   -   an aqueous resin having a hydroxyl group and a carboxyl group        (A2);    -   a melamine resin (D),    -   a weak acid catalyst (E); and    -   an aqueous polyurethane resin (F), wherein

the aqueous resin having a hydroxyl group and a carboxyl group (A1)contained in the aqueous intermediate coating composition has a hydroxylvalue of 80 to 200 mgKOH/g and an acid value of 10 to 40 mgKOH/g interms of resin solid content,

the aqueous resin having a hydroxyl group and a carboxyl group (A2)contained in the aqueous base coating composition has a hydroxyl valueof 80 to 200 mgKOH/g in terms of resin solid content,

the hydrophilicized-modified carbodiimide compound (C) is a compoundrepresented by a formula (I), (II), or (III) below,

[Chemical Formula 1]

YOCONH—X—NHCOO-—Z—OCONH—X—NHCOOY   (I)

wherein each X is a bifunctional organic group having at least onecarbodiimide group, Y is each same or different structure resulting fromelimination of a hydroxyl group from a polyalkylene glycol monoalkylether, and Z is a structure resulting from elimination of a hydroxylgroup from a bifunctional polyol having a number-average molecularweight of 200 to 5,000,

wherein each X is a bifunctional organic group having at least onecarbodiimide group, Y is each same or different structure resulting fromelimination of a hydroxyl group from a polyalkylene glycol monoalkylether, R⁰ is hydrogen, a methyl group or an ethyl group, each R¹ is analkylene group having 4 or less carbon atoms, n is 0 or 1, and each m isa number from 0 to 60,

[Chemical Formula 3]

YOCONH—X—NHCOOY   (III)

wherein X is a bifunctional organic group having at least onecarbodiimide group, and Y is each same or different structure resultingfrom elimination of a hydroxyl group from a polyalkylene glycolmonoalkyl ether,

the melamine resin (D) has an average imino group amount of 1.0 or more,and an average methylol group amount of 0.5 or more, per one melaminenucleus,

a mass ratio of the aqueous resin (A2) and the melamine resin

(D) contained in the aqueous base coating composition is in the range of(A2)/(D)=1 to 3 in terms of solid content,

an amount of the weak acid catalyst (E) contained in the aqueous basecoating composition is within the range of 0.1 to 10.0 parts by mass,based on 100 parts by mass of solid contents ((A2)+(D)) of the aqueousresin (A2) and the melamine resin (D) contained in the aqueous basecoating composition,

the aqueous polyurethane resin (F) has a glass transition point (Tg) of−50° C. or less, and

a cured film of the aqueous polyurethane resin (F) has an elongation atbreak of 400% or more at −20° C.

[2]

The method for forming a multilayer coating film mentioned above,wherein a content of the hydrophilicized-modified carbodiimide compound(C) is 1 to 8% by mass based on a resin solid content of the aqueousintermediate coating composition.

[3]

The method for forming a multilayer coating film mentioned above,wherein a content of the aqueous polyurethane resin (F) is 15% by massor more and 30% by mass or less based on a resin solid content of theaqueous base coating composition.

[4]

The method for forming a multilayer coating film mentioned above,wherein a ratio of an equivalent of a carbodiimide group of thehydrophilicized-modified carbodiimide compound (C) to an equivalent ofan acid group of the aqueous resin (A1) contained in the aqueousintermediate coating composition is 0.1 to 0.6.

[5]

The method for forming a multilayer coating film mentioned above,wherein an amount of the weak acid catalyst (E) is within the range of0.1 to 5.0 parts by mass, based on 100 parts by mass of solid contents((A2)+(D)) of the aqueous resin (A2) and the melamine resin (D)contained in the aqueous base coating composition.

[6]

The method for forming a multilayer coating film mentioned above,wherein the weak acid catalyst (E) comprises a phosphate ester compound.

[7]

The method for forming a multilayer coating film mentioned above,wherein the aqueous base coating composition further comprises anaqueous resin (G) having a hydroxyl value of less than 80 mgKOH/g.

[8]

The method for forming a multilayer coating film mentioned above,wherein the object to be coated includes a steel plate part and a resinpart.

[9]

The method for forming a multilayer coating film mentioned above,wherein the method further comprises a clear coating film formation stepof applying a clear coating composition onto the uncured base coatingfilm obtained in the base coating film formation step to form an uncuredclear coating film, wherein

the curing step is a step of curing the resulting uncured intermediatecoating film, base coating film, and clear coating film by heating.

[10]

The method for forming a multilayer coating film mentioned above,wherein a heating temperature in the curing step is 70 to 120° C.

Advantageous Effect of the Invention

The method for forming a multilayer coating film of the presentinvention is advantageous in that a curing reaction proceeds well evenunder heating conditions of low temperature conditions (e.g., heatingconditions at 100° C. or less), so that a cured coating film havingexcellent coating film properties can be obtained. The method forforming a multilayer coating film of the present invention can be usedsuitably for coating of an object to be coated having a steel plate partand a resin part, which is difficult to be subjected to high-temperatureheat curing treatment but is required to be excellent in coating filmproperties (water resistance, chipping resistance, etc.).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method for forming a multilayer coating film of the presentinvention includes:

an intermediate coating film formation step of applying an aqueousintermediate coating composition to an object to be coated to form anuncured intermediate coating film;

a base coating film formation step of applying an aqueous base coatingcomposition onto a resulting uncured intermediate coating film to forman uncured base coating film; and

a curing step of curing the resulting uncured intermediate coating filmand the base coating film by heating. The method for forming amultilayer coating film of the present invention is characterized inthat by using the above-specified aqueous intermediate coatingcomposition and the above-specified aqueous base coating composition incombination, a multilayer coating film having good water resistance andgood chipping resistance can be obtained even when the aqueous coatingcompositions are baked and cured under low-temperature curingconditions, for example. In the following, the coating compositions tobe used in the respective coating film formation steps are will bedescribed.

Aqueous Intermediate Coating Composition

The aqueous intermediate coating composition to be used in the method ofthe present invention contains an aqueous resin having a hydroxyl groupand a carboxyl group (A1), a polyisocyanate compound (B), and ahydrophilicized-modified carbodiimide compound, (C).

Aqueous Resin having Hydroxyl Group and Carboxyl Group (A1)

The aqueous resin having a hydroxyl group and a carboxyl group (A1) is abinder component that undergoes a curing reaction with thewater-dispersible blocked polyisocyanate compound (B) and thehydrophilicized-modified carbodiimide compound (C). The aqueous resinhaving a hydroxyl group and a carboxyl group (A1) to be used in thepresent invention is required to have:

a hydroxyl value of 80 to 200 mgKOH/g in terms of resin solid content,and

an acid value of 10 to 40 mgKOH/g in terms of resin solid content.

The hydroxyl value in terms of resin solid content may preferably be 80to 160 mgKOH/g, and the acid value in terms of resin solid content maypreferably be 15 to 35 mgKOH/g.

The aqueous resin having a hydroxyl group and a carboxyl group (A1) tobe used in the present invention is high in hydroxyl value as comparedwith its acid value. Inclusion of such an aqueous resin (A1) as well asthe components (B) to (C) offers an advantage that good chippingresistance is obtained even when the aqueous intermediate coatingcomposition is applied and then cured at a low temperature.

The aqueous resin (A1) may be composed of a single resin satisfying theabove requirement regarding a hydroxyl value and an acid value in termsof resin solid content or alternatively may be composed of two or moreresins each satisfying the above requirement regarding a hydroxyl valueand an acid value.

The aqueous resin (A1) has two types of functional groups, namely, ahydroxyl group and a carboxyl group, as reactive groups that participatein curing. In the aqueous intermediate coating composition in thepresent invention, the hydroxyl group of the aqueous resin (A1) reactswith the polyisocyanate compound (B) and the carboxyl group of theaqueous resin (A1) reacts with the hydrophilicized-modified carbodiimidecompound (C).

The aqueous resin (A1) is not particularly limited with respect to itstype as long as it satisfies the requirement regarding a hydroxyl groupand a carboxyl group, but it may preferably be an acrylic resin and/or apolyester resin because these materials are easily produced and easilyavailable. From the viewpoint of adjustment of coating film properties,it may be preferred to use an acrylic resin alone or a mixture of anacrylic resin and a polyester resin as the aqueous resin (A1). Forexample, in use as an intermediate coating composition, it may be morepreferred to use a mixture of an acrylic resin and a polyester resin asthe aqueous resin (A1). For example, in use as a top base coatingcomposition, it may be more preferred to use an acrylic resin as theaqueous resin (A1).

Concerning the acrylic resin that can be used suitably as the aqueousresin (A1), a resin of interest can be obtained, for example, bysubjecting to acrylic copolymerization monomers containing an α,β-ethylenically unsaturated monomer having a hydroxyl group and an α,β-ethylenically unsaturated monomer having a carboxyl group in suchamounts that satisfy the requirement on the hydroxyl value and the acidvalue regarding the hydroxyl group and the carboxyl group.

Examples of the α, β-ethylenically unsaturated monomer having a hydroxylgroup include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl(meth)acrylate, 4-hydroxybutyl (meth)acrylate, allyl alcohol, methacrylalcohol, and an adduct of hydroxyethyl (meth)acrylate andε-caprolactone. Preferred among these may be 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth) acrylate, and an adduct of hydroxyethyl(meth)acrylate and c-caprolactone. In the present description,“(meth)acryl” shall mean both acryl and methacryl.

Examples of the α, β-ethylenically unsaturated monomer having a carboxylgroup include acrylic acid, methacrylic acid, acrylic acid dimer,crotonic acid, 2-acryloyloxyethylphthalic acid,2-acryloyloxyethylsuccinic acid, ω-carboxy-polycaprolactonemono(meth)acrylate, maleic acid, fumaric acid, and itaconic acid.Preferred among these are acrylic acid and methacrylic acid.

In the acrylic copolymerization for obtaining the aqueous resin (A1),other α, β-ethylenically unsaturated monomer can be used if necessary.Examples of the above-described other α, β-ethylenically unsaturatedmonomer include (meth)acrylic acid esters (e.g., methyl (meth)acrylate,ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate,isobutyl (meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl methacrylate, phenyl acrylate, isobornyl (meth)acrylate, cyclohexyl methacrylate, (meth)acrylic acid tert-butylcyclohexyl, dicyclopentadienyl (meth) acrylate, anddihydrodicyclopentadienyl (meth) acrylate), and polymerizable amidecompounds (e.g., (meth)acrylamide, N-methylol(meth)acrylamide, andN-butoxymethyl(meth)acrylamide).

The method for obtaining the aqueous resin (A1) may be a method in whichan acrylic resin is obtained by performing solution polymerization andthen the resulting material is subjected to hydrophilization or a methodin which an emulsion is obtained by performing emulsion polymerizationin an aqueous medium.

When an emulsion is obtained by performing emulsion polymerization, acrosslinking monomer can be used as the above-described other α,β-ethylenically unsaturated monomer. The crosslinking monomer is acompound having two or more radically polymerizable, ethylenicallyunsaturated groups in its molecule, and examples thereof includedivinylbenzene, allyl (meth)acrylate, and ethylene glycoldi(meth)acrylate.

The solution polymerization mentioned above is commonly a method thatinvolves stirring a solvent while dropping thereinto a mixture of α,β-ethylenically unsaturated monomers to be used as raw materialstogether with a polymerization initiator under heating conditions. Theconditions for the solution polymerization may include a polymerizationtemperature of 60 to 160° C. and a dropping time of 0.5 to 10 hours, forexample. The α, β-ethylenically unsaturated monomers to be used as rawmaterials may be polymerized separately in two steps. In this case, theα, β-ethylenically unsaturated monomers to be used as raw materials arerequired as a whole to satisfy the requirement regarding a hydroxylgroup and a carboxyl group.

The polymerization initiator mentioned above is not particularly limitedas long as the polymerization initiator is used for commonpolymerization, and examples thereof include azo compounds andperoxides. Generally, an amount of the polymerization initiator relativeto 100 parts by mass of the monomer mixture is 0.1 to 18 parts by mass,and preferably 0.3 to 12 parts by mass.

The solvent that can be used here is not particularly limited as long asthe solvent does not affect the reaction adversely, and examples thereofinclude alcohols, ketones, ethers, and hydrocarbon solvents. Moreover,in order to adjust the molecular weight, a mercaptan such as laurylmercaptan, or a chain transfer agent such as α-methylstyrene dimer maybe used if necessary.

The acrylic resin thus obtained by solution polymerization maypreferably have a number-average molecular weight of 4,000 to 20,000. Inthe present description, the number-average molecular weight of theacrylic resin obtained by solution polymerization can be measured by gelpermeation chromatography (GPC) using a polystyrene standard sample.

The acrylic resin has a glass transition point (Tg) of preferably withina range of −20 to 80° C. The glass transition point of an acrylic resincan be determined by calculation from the type and amount of themonomers used for the preparation of the acrylic resin. The glasstransition point of the acrylic resin may be measured with adifferential scanning calorimeter (DSC).

The acrylic resin resulting from the solution polymerization describedabove is subjected to removal of the solvent if necessary, and then abasic material is added thereto and the resulting material is subjectedto hydrophilization, so that the aqueous resin (A1) is obtained.Examples of the basic compound include ammonia, methylamine, ethylamine,dimethylamine, diethylamine, trimethylamine, triethylamine,dimethylethanolamine, diethanolamine, diethylaminoethanol, andtriethanolamine. An amount of the basic compound to be added maypreferably be adjusted such that a neutralization ratio relative to thecarboxyl groups that the acrylic resin resulting from the solutionpolymerization has is 60 to 100%. When the neutralization ratio is lessthan 60%, the hydrophilization is insufficient and the storage stabilitymay be poor. A resin solid content of the thus-obtained aqueous resin(A1) is commonly adjusted to 25 to 55% by mass.

The thus-obtained acrylic resin can be used in the form of an aqueousacrylic dispersion. Such an aqueous acrylic dispersion may preferablyhave a volume-average particle diameter within a range of 0.01 to 1 μm.The fact that the volume-average particle diameter is within the aboverange offers an advantage that the stability of the aqueous dispersionis improved and the appearance of a resulting coating film is alsoimproved. The same applies to an acrylic emulsion described below, andthe volume-average particle diameter can be adjusted through theadjustment of the monomer composition and/or the emulsion polymerizationconditions.

When emulsion polymerization in an aqueous medium is performed duringthe preparation of the aqueous resin (A1), the polymerization can becarried out, for example, by dissolving an emulsifier in the aqueousmedium containing water and, if necessary, an organic solvent such as analcohol, and dropping a polymerization initiator and a mixture of the α,β-ethylenically unsaturated monomers to be used as raw materials withstirring under heat. The mixture of the α, β-ethylenically unsaturatedmonomers to be used as raw materials may be emulsified in advance usingan emulsifier and water.

Examples of the polymerization initiator that can be suitably used foremulsion polymerization include lipophilic azo compounds (e.g.,azobisisobutyronitrile, 2,2′-azobis(2-methylbutyronitrile), and2,2′-azobis(2,4-dimethylvaleronitrile)); hydrophilic azo compounds(e.g., 4,4′-azobis(4-cyanovaleric acid) and2,2-azobis(N-(2-carboxyethyl)-2-methylpropionamidine), which areanionic, and 2,2′-azobis(2-methylpropionamidine), which is cationic);redox-type lipophilic peroxides (e.g., benzoyl peroxide,parachlorobenzoyl peroxide, lauroyl peroxide, and tert-butylperbenzoate); and redox-type hydrophilic peroxides (e.g., potassiumpersulfate and ammonium persulfate).

As the emulsifier, common emulsifiers that a person skilled in the artusually uses can be used. Particularly preferred as the emulsifier maybe reactive emulsifiers, e.g., Antox MS-60 (produced by Nippon NyukazaiCo., Ltd.), Eleminol JS-2 (produced by Sanyo Chemical Industries, Ltd.),ADEKA REASOAP NE-20 (produced by ADEKA, Inc.), Aqualon HS-10 (producedby Dai-Ichi Kogyo Seiyaku Co., Ltd.), and LATEMUL PD-104 (produced byKao Corporation). Moreover, in order to adjust the molecular weight,mercaptan such as lauryl mercaptan, or a chain transfer agent such asa-methylstyrene dimer may be used if necessary.

A reaction temperature is determined depending on an initiator, and forexample, the reaction temperature is 60 to 90° C. for azo initiators orperoxides and may preferably be 30 to 70° C. for redox type initiators.Generally, a reaction time is 1 to 8 hours. Generally, an amount of theinitiator relative to 100 parts by mass of the monomer mixture is 0.1 to5% by mass. The emulsion polymerization may be performed in multiplesteps, for example, in two steps. That is, a portion of the mixture ofthe α, β-ethylenically unsaturated monomers to be used as raw materialsis subjected to emulsion polymerization, and then the remainder of theα, β-ethylenically unsaturated monomer mixture is added thereto andsubjected to further emulsion polymerization.

From the viewpoint of storage stability, the emulsion can be used at pH5 to 10 through neutralization with a basic compound. The basic compoundmay be the same as that to be used in the hydrophilization of theacrylic resin obtained in the preceding solution polymerization. Theneutralization may preferably be carried out by adding theaforementioned basic compound to the system before or after the emulsionpolymerization.

When an acrylic emulsion is used as the aqueous resin (A1), the acrylicemulsion may preferably have a number-average molecular weight of 10,000to 80,000. The fact that the acrylic emulsion has a hydroxyl value of 80to 200 mgKOH/g, an acid value of 10 to 40 mgKOH/g and a number-averagemolecular weight within a range of 10,000 to 80,000 offers an advantagethat coating material stability is kept good and the crosslinkingdensity in a resulting coating film falls into a better range. This isconsidered to be because the low-temperature curability of thepolyisocyanate compound (B) that reacts with the hydroxyl groups of theaqueous resin (A1) is secured due to the fact that the range of thenumber-average molecular weight is a relatively high range of 10,000 to80,000 and the acrylic emulsion has hydroxyl groups as many as shown bythe above range, and thus the crosslinking density in a resultingcoating film will fall within a better range.

The number-average molecular weight of the acrylic emulsion can bemeasured by gel permeation chromatography (GPC) using a polystyrenestandard sample after removing moisture by reduced pressure drying orthe like.

The aqueous resin (A1) may contain a polyester resin. Generally, thepolyester resin that can be used as the aqueous resin (A1) can beprepared by condensing a polyhydric alcohol component and a polybasicacid component such that the requirement regarding a hydroxyl group anda carboxyl group will be satisfied.

Examples of the polyhydric alcohol component may includehydroxycarboxylic acid components such as ethylene glycol, diethyleneglycol, propylene glycol, 1,3-propanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 2,2-diethyl-1,3-propanediol, neopentylglycol, 1,9-nonanediol, 1,4-cyclohexanediol, neopentyl glycolhydroxypivalate ester, 2-butyl-2-ethyl-1,3-propanediol,3-methyl-1,5-pentanediol, and 2,2,4-trimethylpentanediol.

Examples of the polybasic acid component may include polybasic acidcomponents and anhydrides thereof such as aromatic polycarboxylic acidsand anhydrides including phthalic anhydride, isophthalic acid,terephthalic acid, trimellitic anhydride, tetrachlorophthalic anhydride,and pyromellitic anhydride; alicyclic polycarboxylic acids andanhydrides thereof including hexahydrophthalic anhydride,tetrahydrophthalic anhydride, and 1,4- and 1,3-cyclohexanedicarboxylicacids; aliphatic polycarboxylic acids and anhydrides thereof includingmaleic anhydride, fumaric acid, succinic anhydride, adipic acid, andsebacic acid. A monobasic acid such as benzoic acid or tert-butylbenzoicacid may be used together, if necessary.

Moreover, monohydric alcohols, monoepoxide compounds such as CARDURA E(trade name, produced by Shell Chemical), and lactones (β-propiolactone,dimethylpropiolactone, butyrolactone, γ-valerolactone, ε-caprolactone,γ-caprolactone, etc.) may be used together as reaction components.

In addition to the above-mentioned components, fatty acids such ascastor oil and dehydrated castor oil, and an oil component that is amixture of one or two or more of such fatty acids may be added to theacid component and the alcohol component. Moreover, it is also possibleto graft an acrylic resin or vinyl resin or to react a polyisocyanatecompound as long as the requirement regarding a hydroxyl group and acarboxyl group is satisfied.

The thus-obtained polyester resin may preferably have a number-averagemolecular weight of 500 to 20,000, and more preferably 1,500 to 10,000.When the number-average molecular weight is less than 500, the storagestability may deteriorate in the case where the polyester resin isdispersed in water. When the number average molecular weight exceeds20,000, the viscosity of the polyester resin increases, and therefore,the solid concentration decreases when the polyester resin is famed intoa coating composition and coating workability may deteriorate.

The polyester resin may preferably have a glass transition point of −20to 80° C. When the glass transition point is less than −20° C., thehardness of a resulting coating film may decrease, and when exceeding80° C., the base hiding property may deteriorate. The glass transitionpoint may more preferably be 0 to 60° C. The glass transition point ofthe polyester resin can be determined by calculation from the type andamount of the monomers used for the preparation of the polyester resin,as in the case of the acrylic resin. The glass transition point of thepolyester resin may be measured with a differential scanning calorimeter(DSC).

The aqueous resin (A1) can be obtained by neutralizing the thus-obtainedpolyester resin with any of the basic compounds mentioned previously.

A content of the aqueous resin (A1) contained in the aqueousintermediate coating composition in the present invention may preferablybe 30 to 80% by mass, more preferably 50 to 80% by mass based on theresin solid content of the aqueous intermediate coating composition.

For example, when the aqueous coating composition is used as anintermediate coating composition and a mixture of an acrylic resin and apolyester resin is used as the aqueous resin (A1), a ratio of theacrylic resin and the polyester resin may preferably be within a rangeof acrylic resin/polyester resin=7/1 to 0.5/1, and more preferablywithin a range of 6/1 to 1/1.

Polyisocyanate Compound (B)

The aqueous intermediate coating composition in the present inventioncontains two components, namely, the polyisocyanate compound (B) and thehydrophilicized-modified carbodiimide compound (C), as components forcuring the aqueous resin (A1). Here, the polyisocyanate compound (B) maybe water-dispersible or hydrophobic. Even when it is hydrophobic, waterdispersibility is secured by the interaction with thehydrophilicized-modified carbodiimide compound (C) which is excellent inwater dispersibility as described below.

Examples of the polyisocyanate compound (B) that is hydrophobic includepolyisocyanate compounds such as aromatic diisocyanates includingtolylene diisocyanate (TDI), 4,4′ -diphenylmethane diisocyanate (MDI),xylylene diisocyanate (XDI), and metaxylylene diisocyanate (MXIDI);aliphatic diisocyanates including hexamethylene diisocyanate (HDI);alicyclic diisocyanates including isophorone diisocyanate (IPDI) andhydrogenated MDI; compounds resulting from such diisocyanate compoundsby reducing their volatility and thereby converting them into less toxicforms; adducts of such diisocyanate compounds, including biurets,uretdiones, and isocyanurates; and relatively low-molecular-weighturethane prepolymers.

On the other hand, examples of the polyisocyanate compound (B) that iswater-dispersible include products prepared by introducing a hydrophilicgroup into the polyisocyanate compounds mentioned above, and productsprepared by mixing and emulsifying a surfactant and therebyself-emulsifying the polyisocyanate compounds.

Examples of the hydrophilic group include anionic groups such as acarboxyl group and a sulfonic acid group, cationic groups such as atertiary amino group, and nonionic groups such as a polyoxyalkylenegroup. Among these, in consideration of the water resistance of aresulting coating film, the hydrophilic group may preferably be anonionic group. As a specific nonionic group, a polyoxyethylene grouphaving high hydrophilicity may be preferable.

Examples of the surfactant suitably used for the preparation of aself-emulsifiable polyisocyanate compound obtained by mixing andemulsifying the above-mentioned polyisocyanate compound and thesurfactant include an anionic surfactant having an anionic group such asa carboxyl group or a sulfonic acid group, a cationic surfactant havinga cationic group such as a tertiary amino group, and a nonionicsurfactant having a nonionic group such as a polyoxyalkylene group.Among them, in consideration of the water resistance of a resultingcoating film, it is more preferable to use a nonionic surfactant.

A commercially available product may be used as the polyisocyanatecompound (B) that is water-dispersible. Examples of the commerciallyavailable products include Aquanate 100, Aquanate 110, Aquanate 200 andAquanate 210 (produced by Tosoh CoLporation), Bayhydur TPLS-2032,SBU-Isocyanate L801, Bayhydur VPLS-2319, Bayhydur 3100, VPLS-2336 andVPLS-2150/1, Bayhydur 305, Bayhydur XP-2655 (produced by Sumika BayerUrethane Co., Ltd.), Takenate WD-720, Takenate WD-725 and TakenateWD-220 (produced by Mitsui Chemicals, Inc.), and RESAMINE D-56 (producedby Dainichiseika Color & Chemicals Mfg. Co., Ltd.).

In the present invention, the polyisocyanate compound (B) to be used maymore preferably be a polyisocyanate compound that is water-dispersible.The polyisocyanate compound (B) may be used singly, or two or morespecies thereof may be used in combination.

A content of the polyisocyanate compound (B) contained in the aqueousintermediate coating composition in the present invention may preferablybe 5 to 55% by mass, more preferably 10 to 45% by mass based on theresin solid content of the aqueous intermediate coating composition.

Hydrophilicized-Modified Carbodiimide Compound (C)

The hydrophilicized-modified carbodiimide compound (C) contained in theaqueous intermediate coating composition in the present invention has,in its molecule, one or a plurality of structural units represented by

—OCONH—X—NHCOOY

wherein X is a bifunctional organic group having at least onecarbodiimide group, and Y is a structure resulting from elimination of ahydroxyl group from a polyalkylene glycol monoalkyl ether. It isconsidered that the inclusion of the structural unit offers bothexcellent dispersibility in water and excellent curability.

The hydrophilicized-modified carbodiimide compound (C) has three types,namely, a compound having one unit, a compound having two units, and acompound having three units of the structural unit shown above.

One example of the compound having two units of the structural unitshown above is a compound represented by the following formula (I).

[Chemical Formula 4]

YOCONH—X—NHCOO—Z—OCONH—X—NHCOOY   (I)

In the above formula (I), each X is a bifunctional organic group havingat least one carbodiimide group, Y is each same or different structureresulting from elimination of a hydroxyl group from a polyalkyleneglycol monoalkyl ether, and Z is a structure resulting from eliminationof a hydroxyl group from a bifunctional polyol having a number-averagemolecular weight of 200 to 5,000.

Here, X can be represented by the following formula (a).

[Chemical Formula 5]

—R²—(—N═C═N—R²—)—_(p)   (a)

In the above formula (a), each R² may preferably be a hydrocarbon grouphaving 6 to 15 carbon atoms. Specific examples of the hydrocarbon groupmay include a phenylene group, a diphenylenemethyl group, adiphenylene(dimethyl)methyl group, a methylphenylene group, adimethylphenylene group, a tetramethylxylylene group, a hexylene group,a cyclohexylene group, and a dicyclohexylenemethyl group. Preferred maybe a dicyclohexylenemethyl group. In the above formula, p is 1 to 10. pis the number of the carbodiimide groups existing in the abovestructural unit, and p may preferably be 2 or more in terms ofcurability, and the upper limit may preferably be 8 or less.

In the present description, repeat numbers, including the above p, arerepresented as average values.

The above Y can be represented by the following formula (b) or (c).

In the above formulas (b) and (c), R³ may preferably be an alkyl grouphaving 1 to 20 carbon atoms. Specific examples thereof include a methylgroup, an ethyl group, a butyl group, a hexyl group, an octyl group, adecyl group, a dodecyl group, and a stearyl group. R⁴ is a hydrogen atomor a methyl group, and may preferably be a hydrogen atom. q is a numberfrom 4 to 40. In the above formulas (b) and (c), when R⁴ is hydrogen,the formulas (b) and (c) represent the same structure.

The above Z is polymeric structure composed of an ether linkage, anester linkage, or a carbonate linkage, and it is difficult to express Zby a general formula. In this regard, see the explanation for abifunctional polyol having 200 to 5,000 that is described below.

A hydrophilicized-modified carbodiimide compound (C) having two units ofthe above structural unit can be obtained by reacting a raw materialcarbodiimide compound having at least two isocyanate groups in itsmolecule with a bifunctional polyol having hydroxyl groups at itsmolecular ends and having a number-average molecular weight of 200 to5,000 in such a ratio that the molar amount of the isocyanate groups ofthe raw material carbodiimide compound is larger than the molar amountof the hydroxyl groups of the polyol, and then further reacting thethus-obtained reaction product with a polyalkylene glycol monoalkylether.

From the viewpoint of reactivity, the raw material carbodiimide compoundhaving at least two isocyanate groups in its molecule may preferablyhave isocyanate groups at its both ends. A method for producing the rawmaterial carbodiimide compound having isocyanate groups at its both endsis well known to those skilled in the art and, for example, acondensation reaction accompanied by a decarboxylation of an organicdiisocyanate can be utilized.

As to the organic diisocyanate, specifically, aromatic diisocyanates,aliphatic diisocyanates, alicyclic diisocyanates, and mixtures thereofcan be used, and specific examples thereof include 1,5-naphthylenediisocyanate, 4,4-diphenylmethane diisocyanate,4,4-diphenyldimethylmethane diisocyanate, 1,3-phenylene diisocyanate,1,4-phenylene diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylenediisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylenediisocyanate, hexamethylene diisocyanate, cyclohexane-1, 4-diisocyanate,xylylene diisocyanate, isophorone diisocyanate,dicyclohexylmethane-4,4-diisocyanate, methylcyclohexane diisocyanate,and tetramethylxylylene diisocyanate. From the viewpoint of reactivity,dicyclohexylmethane-4,4-diisocyanate may be preferred.

For the condensation reaction, a carbodiimidization catalyst is usuallyused. Specific examples of the carbodiimidization catalyst includephospholene oxides such as 1-phenyl-2-phospholene-1-oxide,3-methyl-2-phospholene-1-oxide, 1-ethyl-2-phospholene-1-oxide,3-methyl-1-phenyl-2-phospholene-1-oxide, and 3-phospholene isomersthereof. From the viewpoint of reactivity,3-methyl-1-phenyl-2-phospholene-1-oxide may be preferred.

While the number-average molecular weight of the bifunctional polyolhaving hydroxyl groups at its both molecule ends is not particularlylimited, it may preferably be 200 to 5,000 from the viewpoint ofreaction efficiency. Specific examples of the bifunctional polyol havinghydroxyl groups at its both molecule ends may include polyether diols,polyester diols, and polycarbonate diols. For example, polyalkyleneglycols such as polyethylene glycol, polypropylene glycol, polyethylenepropylene glycol, polytetramethylene ether glycol, polyhexamethyleneether glycol, and polyoctamethylene ether glycol; polyester diols suchas polyethylene adipate, polybutylene adipate, polyhexamethyleneadipate, polyneopentyl adipate, poly-3-methylpentyl adipate,polyethylene/butylene adipate, and polyneopentyl/hexyl adipate;polylactone diols such as polycaprolactone diol andpoly-3-methylvalerolactone diol; polycarbonate diols such aspolyhexamethylenecarbonate diol, and mixtures thereof can be mentioned.

The reaction of the raw material carbodiimide compound having at leasttwo isocyanate groups in its molecule with the bifunctional polyolhaving hydroxyl groups at its molecular ends and having a number-averagemolecular weight of 200 to 5,000 is performed by reacting them in such aratio that the molar amount of the isocyanate groups of the raw materialcarbodiimide compound is larger than the molar amount of the hydroxylgroups of the polyol. When the molar amount of the isocyanate groups issmaller than or equal to the molar amount of the hydroxyl groups, areaction together with a polyalkylene glycol monoalkyl ether describedbelow cannot be performed sufficiently.

The ratio between the molar amount of the isocyanate groups of the rawmaterial carbodiimide compound and the molar amount of the hydroxylgroups of the polyol having hydroxyl groups at its molecular ends maypreferably be 1.1:1.0 to 2.0:1.0 from the viewpoint of reactionefficiency and economic efficiency. A degree of polymerization of theraw material carbodiimide compound and the bifunctional polyol havinghydroxyl groups at its both molecular ends in a reaction productobtained via this step may preferably be 1 to 10 from the viewpoint ofreaction efficiency.

By further reacting the thus-obtained reaction product with apolyalkylene glycol monoalkyl ether, a hydrophilicized-modifiedcarbodiimide compound (C) having two units of the above structural unitcan be obtained. As the polyalkylene glycol monoalkyl ether, apolyalkylene glycol monoalkyl ether represented by the following formula(b′) or (c′) is used.

In the above formulas (b′) and (c′), the contents described for R³, R⁴,and q in the preceding formulas (b) and (c) apply as they are. The typeof R⁴ and q in the above unit are set appropriately within the aboveranges, respectively, in consideration of storage stability,dispersibility in water, and reactivity after volatilization of water.It may be preferable from the viewpoint of dispersibility in water thatR³ in the monoalkoxypolyalkylene glycol be a methyl group and R⁴ be ahydrogen atom. Moreover, from the viewpoint of dispersibility in waterand reactivity after volatilization of water, the q may preferably be 4to 20, and more preferably 6 to 12.

As the polyalkylene glycol monoalkyl ether, a polyalkylene glycolmonoalkyl ether having a number-average molecular weight of 200 to 5,000may preferably be used. The alkyl group of the polyalkylene glycolmonoalkyl ether may preferably be an alkyl group having 1 to 20 carbonatoms. Specific examples of the polyalkylene glycol monoalkyl etherinclude those composed of polyethylene glycol, polypropylene glycol, ormixtures thereof each of which is capped at one end with an alkyl grouphaving 1 to 20 carbon atoms. More detailed specific examples of such apolyalkylene glycol monoalkyl ether include polyethylene glycolmonomethyl ether, polyethylene glycol mono-2-ethylhexyl ether,polyethylene glycol monolauryl ether, polypropylene glycol monomethylether, polypropylene glycol mono-2-ethylhexyl ether, and polypropyleneglycol monolauryl ether, each having a number-average molecular weightof 200 to 5,000.

The reaction product and the polyalkylene glycol monoalkyl ether arereacted in such a ratio that the molar amount of the isocyanate groupsof the reaction product is equal to or larger than the molar amount ofthe hydroxyl groups of the polyalkylene glycol monoalkyl ether. When themolar amount of the isocyanate groups is smaller than the molar amountof the hydroxyl groups, the reaction of the polyalkylene glycolmonoalkyl ether with the reaction product cannot be carried outsufficiently. The molar amount of the isocyanate groups of the reactionproduct can be measured directly, and a value calculated from thecharging formulation may be adopted.

In the reaction of the raw material carbodiimide compound with thebifunctional polyol having hydroxyl groups at its molecular ends and thereaction of the reaction product with the polyalkylene glycol monoalkylether, a catalyst may be used. The temperature during the reactions isnot particularly limited, and from the viewpoint of control of thereaction system and reaction efficiency, the temperature may preferablybe 60 to 120° C. In addition, an organic solvent free from activehydrogen may preferably be used in the reactions.

Such a two-step reaction can provide a hydrophilicized-modifiedcarbodiimide compound (C) having two units of the above structural unit.The thus-produced hydrophilicized-modified carbodiimide compound (C)does not have only the structure of the formula (I) provided above, butis a mixture containing other various reaction products derived from theraw materials used. Generally, however, it may be considered to have thestructure of the above formula (I).

One example of the hydrophilicized-modified carbodiimide compound (C)having three units of the above structural unit is a compoundrepresented by the following formula (II).

In the above formula (II), for X and Y, the description for X and Y madefor the preceding one having two units of the above structural unit canapply as it is. R⁰ is hydrogen, a methyl group, or an ethyl group. EachR¹ is an alkylene group having 4 or less carbon atoms, and may be eithersame or different. Specific alkylene groups include a methylene group,an ethylene group, a propylene group, and a butylene group. n is 0 or 1,and m is a number from 0 to 60.

R⁰ , R¹, n and m are determined depending on a trifunctional polyol tobe used for the production of the hydrophilicized-modified carbodiimidecompound (C).

When m is 11 or more, the ratio of a hydrophilic section to ahydrophobic section may preferably be 2.0 to 6.3. The ratio of thehydrophilic section to the hydrophobic section can be determined bydividing the molecular weight of the moiety of an oxymethylene group oran oxyethylene group existing in the carbodiimide compound by themolecular weight of the carbodiimide compound.

The hydrophilicized-modified carbodiimide compound (C) having threeunits of the above structural unit can be obtained by reacting a rawmaterial carbodiimide compound having at least two isocyanate group inone molecule with a polyalkylene glycol monoalkyl ether in such a ratiothat the equivalent of the isocyanate groups of the raw materialcarbodiimide compound is larger than the equivalent of the hydroxylgroups of the polyalkylene glycol monoalkyl ether, and further reactingthe resulting reaction product with a trifunctional polyol.

For the raw material carbodiimide compound having at least twoisocyanate group in one molecule, the description made for the rawmaterial carbodiimide compound of the hydrophilicized-modifiedcarbodiimide compound (C) having two units of the above structural unitapplies as it is.

The reaction of the raw material carbodiimide compound with thepolyalkylene glycol monoalkyl ether is required to make isocyanategroups remain in order to further react with a trifunctional polyolafter the reaction. For this reason, it is necessary in the abovereaction that the equivalent of the isocyanate groups is larger than theequivalent of the hydroxyl groups, and it may be preferred that theequivalent ratio of the isocyanate groups to the hydroxyl groups be 2/1.The reaction can usually be carried out under conditions well known tothose skilled in the art, and a tin-based catalyst may be used, ifnecessary.

For the polyalkylene glycol monoalkyl ether, the description made forthe polyalkylene glycol monoalkyl ether of the hydrophilicized-modifiedcarbodiimide compound (C) having two units of the above structural unitapplies as it is.

Next, the thus-obtained reaction product is reacted with a trifunctionalpolyol. The amount of the trifunctional polyol to be used for thereaction may preferably be such an amount that the hydroxyl groupequivalent is equal to or larger than the isocyanate equivalent in thereaction product, and more preferably, the isocyanate equivalent isequal to the hydroxyl group equivalent. The isocyanate equivalent in thereaction product not only can be measured directly but also can bedetermined by calculation from the blending ratio of the diisocyanatecompound and the polyalkylene glycol monoalkyl ether in the precedingstep. The reaction can be carried out in the same manner as the reactionof the raw material carbodiimide compound with the polyalkylene glycolmonoalkyl ether described previously.

The trifunctional polyol may preferably be trimethylolpropane, glycerol,or an alkylene oxide adduct of these because of its easy availability.Examples of the alkylene oxide include ethylene oxide and propyleneoxide. An alkylene oxide adduct of glycerol is commercially availablefrom Sanyo Chemical Industries, Ltd. as GP Series. In consideration ofthe curing reactivity of a three-chain type hydrophilic carbodiimidecompound to be obtained, one is particularly preferred in which alkyleneoxide has been added to every hydroxyl group. Of the aforementioned GPSeries, GP-250 and GP-3000 are mentioned as those having such astructure.

Such a two-step reaction can provide a hydrophilicized-modifiedcarbodiimide compound (C) having three units of the above structuralunit. The thus-produced hydrophilicized-modified carbodiimide compound(C) does not have only the structure of the formula (II) describedabove, but it may be considered to have the structure of the aboveformula (II).

One example of the hydrophilicized -modified carbodiimide compound (C)having one unit of the above structural unit is a compound representedby the following formula (III),

[Chemical Formula 9]

YOCONH—X—NHCOOY   (III)

wherein X is a bifunctional organic group having at least onecarbodiimide group, and Y is each same or different structure resultingfrom elimination of a hydroxyl group from a polyalkylene glycolmonoalkyl ether.

X in the formula (III) is a group that can be represented by formula (a)in the above formula (I).

Y in the formula (III) is a structure resulting from elimination ofhydroxyl groups from same or different polyethylene glycol monoalkylethers. The Y can represent the same structure of the Y in theabove-described formula (I). Use of the hydrophilicized-modifiedcarbodiimide compound (C) represented by the formula (III) offers anadvantage that a crosslinking density can be held at a higher level.Conceivable reasons for this are that in the formulas (I) and (II) inwhich there are a plurality of carbodiimide units, the efficiency of thereaction with an acid is low under a low acid value of an aqueous resinand that the crosslinking of the hydroxyl groups of the aqueous resinand the isocyanate is not disturbed because the formula (III) does nothave a bulky structure unlike the formulas (I) and (II). Accordingly, itis considered that the crosslinking density of thehydrophilicized-modified carbodiimide compound (C) represented by theformula (III) becomes high for these reasons.

The Y in the formula (III) may preferably be same or different structureselected from the following (i) or (ii):

-   (i) a structure resulting from elimination of a hydroxyl group from    a polyethylene glycol monoalkyl ether in which an alkyl group having    1 to 3 carbon atoms is ether-linked to an end of a polyethylene    oxide unit having a repeat number of 6 to 20,-   (ii) a structure resulting from elimination of a hydroxyl group from    a polypropylene glycol monoalkyl ether in which an alkyl group    having 1 to 8 carbon atoms is ether-linked to an end of a    polypropylene oxide unit having a repeat number of 4 to 60.

More preferably, the repeat number of the polypropylene oxide units ofthe above (ii) is 15 to 60.

Use of the hydrophilicized-modified carbodiimide compound (C)represented by the formula (III) and having the above (i) and (ii)offers an advantage that excellent dispersibility in water is attainedand stability is improved and crosslinking density is held at a higherlevel.

The hydrophilicized-modified carbodiimide compound (C) represented bythe formula (III) can be prepared by reacting same or differentpolyalkylene glycol monoalkyl ethers with the raw material carbodiimidecompound obtained through the above-described condensation reactionaccompanied by decarbonization of an organic diisocyanate.

The polyalkylene glycol monoalkyl ether may more preferably be

a polyethylene glycol monoalkyl ether in which an alkyl group having 1to 3 carbon atoms is ether-linked to an end of a polyethylene oxide unithaving a repeat number of 6 to 20, or

a polypropylene glycol monoalkyl ether in which an alkyl group having 1to 8 carbon atoms is ether-linked to an end of a polypropylene oxideunit having a repeat number of 4 to 60. In the preparation of thehydrophilicized-modified carbodiimide compound (C) represented by theformula (III), such a polyethylene glycol monoalkyl ether and such apolypropylene glycol monoalkyl ether may be used either singly or incombination.

Specific examples of the polyethylene glycol monoalkyl ether includepolyethylene glycol monomethyl ether, polyethylene glycol monoethylether, and polyethylene glycol monopropyl ether, and especially,polyethylene glycol monomethyl ether is suitable.

Specific examples of the polypropylene glycol monoalkyl ether includepolypropylene glycol monomethyl ether, polypropylene glycol monoethylether, polypropylene glycol monobutyl ether, and polypropylene glycol2-ethylhexyl ether, and especially, polypropylene glycol monobutyl etheris suitable.

In the hydrophilicized-modified carbodiimide compound (C) represented bythe above formula (III), it may be preferable that one Y is (i) and theother Y is (ii), and the ratio of (i) a structure resulting fromelimination of a hydroxyl group from a polyethylene glycol monoalkylether in which an alkyl group having 1 to 3 carbon atoms is ether-linkedto an end of a polyethylene oxide unit having a repeat number of 6 to 20and (ii) a structure resulting from elimination of a hydroxyl group froma polypropylene glycol monoalkyl ether in which an alkyl group having 1to 8 carbon atoms is ether-linked to an end of a polypropylene oxideunit having a repeat number of 4 to 60 is within a range of (i):(ii)=1:0.7 to 1:8.

In the hydrophilicized-modified carbodiimide compound (C) represented bythe formula (III), it may be preferable that the surrounding of thecarbodiimide group is hydrophobic to a certain degree in order toenhance water resistance when a coating film is formed. Moreover, inorder to suppress the deactivation of carbodiimide by water and to keepstability, it may be preferable that the surrounding of the carbodiimidegroup is hydrophobic to a certain degree and the contact with watermolecules is kept low. On the other hand, in thehydrophilicized-modified carbodiimide compound (C) represented by theformula (III), the compound is required to have a polyethylene glycolstructure in a certain amount in order to maintain hydrophilicity. Whenthe above structures (i) and (ii) are present in a ratio within therange of (i):(ii)=1:0.7 to 1:8, the hydrophobicity can be kept at acertain degree at the surrounding of the carbodiimide group, while thehydrophilicity of the carbodiimide compound is secured. This offers anadvantage that an aqueous intermediate coating composition superior inlow-temperature curability and also superior in coating materialstability can be obtained. The ratio (i):(ii) may more preferably bewithin the range of (i):(ii)=1:0.7 to 1:1.5.

A content of the hydrophilicized-modified carbodiimide compound (C)contained in the aqueous intermediate coating composition is preferably1 to 8% by mass based on the resin solid content of the aqueousintermediate coating composition The fact that the amount of thehydrophilicized-modified carbodiimide compound (C) is within the aboverange offers an advantage that good water resistance and goodwater-resistant shrinkage can be obtained in a resulting multilayercoating film.

Preparation, etc. of Aqueous Intermediate Coating Composition

The aqueous intermediate coating composition of the present inventioncomprises an aqueous resin having a hydroxyl group and a carboxyl group(A1), a polyisocyanate compound (B), and a hydrophilicized -modifiedcarbodiimide compound (C).

In the above aqueous intermediate coating composition, a ratio of theequivalent of the carbodiimide group included in thehydrophilicized-modified carbodiimide compound (C) to the equivalent ofthe isocyanate group included in the polyisocyanate compound (B) maypreferably be within the range of 0.01 to 0.20. Thus, in the presentinvention is characterized in that the equivalent of the carbodiimidegroup is very small based on the equivalent of the isocyanate group. Inthe above aqueous intermediate coating composition, the fact that theratio of the equivalent of the carbodiimide group to the equivalent ofthe isocyanate group is within the range of 0.01 to 0.20 offers anadvantage that the crosslinking density of a resulting coating film isincreased while the low-temperature curability is secured and goodcoating film properties are secured. The equivalent ratio may morepreferably be in the range of 0.01 to 0.09.

The aqueous resin having a hydroxyl group and a carboxyl group (A1) tobe used in the present invention has a hydroxyl value of 80 to 200mgKOH/g and an acid value of 10 to 40 mgKOH/g in terms of resin solidcontent as described above. That is, it is characterized in that thehydroxyl value is much greater than the acid value. The fact that theratio of the equivalent of the carbodiimide group included in thehydrophilicized-modified carbodiimide compound (C) to the equivalent ofthe isocyanate group included in the polyisocyanate compound (B) iswithin the range of 0.01 to 0.20 in addition to the use of such anaqueous resin (A1) offers an advantage that a coating film having asufficient crosslinking density can be obtained while securing coatingmaterial stability. For example, when the ratio of the equivalent of thecarbodiimide group included in the hydrophilicized-modified carbodiimidecompound (C) to the equivalent of the isocyanate group included in thepolyisocyanate compound (B) is merely reduced without using the aqueousresin (A1) described above, coating material stability may be greatlyreduced. This is because the stability of the polyisocyanate compound(B) in the coating composition is improved due to the existence of thehydrophilicized-modified carbodiimide compound (C) in the aqueousintermediate coating composition.

The present invention is characterized by using an aqueous resin (A1)having a hydroxyl value greatly higher than an acid value. Due to thefact that the aqueous resin (A1) has such a high hydroxyl value, a highcrosslinking density will be achieved in a resulting coating film. Inaddition, due to the fact that the acid value of the aqueous resin (A1)is low, an undesirable side reaction which may occur between an acidgroup of the aqueous resin (A1) and an isocyanate group of thepolyisocyanate compound (B) will be suppressed. Moreover, the fact thatthe ratio of the equivalent of the carbodiimide group included in thehydrophilicized-modified carbodiimide compound (C) to the equivalent ofthe isocyanate group included in the polyisocyanate compound (B) iswithin the range of 0.01 to 0.20 and the fact that the amount of thecarbodiimide group is extremely small offer an advantage that asufficient crosslinking density is achieved even after storage of acoating composition.

In the aqueous intermediate coating composition, the ratio of theequivalent of the isocyanate group included in the polyisocyanatecompound (B) to the equivalent of the hydroxyl group contained in theaqueous resin (A1) may preferably be within the range of 0.6 to 1.5. Thehydroxyl group of the aqueous resin (A1) and the isocyanate group of thepolyisocyanate compound (B) are groups which react with each other. Thefact that the equivalent ratio of these groups is within theabove-mentioned range offers an advantage that a curing reactionproceeds satisfactorily even at a low temperature and, and thus, acoating film having a desirable crosslinking density can be obtained.

In the aqueous intermediate coating composition, the ratio of theequivalent of the carbodiimide group included in thehydrophilicized-modified carbodiimide compound (C) to the equivalent ofthe acid group included in the aqueous resin (A1) may preferably bewithin the range of 0.1 to 1.0. The equivalent ratio may more preferablybe within the range of 0.1 to 0.6. In this case, with respect to theequivalents of the carbodiimide group and the acid group which reactwith each other, the acid group is present in an excess amount. Thisprovides a state where basically no carbodiimide groups remain in acured coating film to be famed and will allow acid groups to remain, andthus offers an advantage that coating film adhesion to an object to becoated is improved.

If necessary, the aqueous intermediate coating composition may contain,in addition to the above-mentioned components (A1) to (C), a pigment, acuring catalyst, a surface conditioner, an antifoaming agent, a pigmentdispersing agent, a plasticizer, a film-forming assistant, anultraviolet absorber, an antioxidant, solvent (water, organic solvents),etc. Since the aqueous intermediate coating composition is excellent inreactivity at low temperatures, it may preferably be produced at acoating site. The aqueous intermediate coating composition can beobtained by mixing the components (A1) to (C).

In the aqueous intermediate coating composition, thehydrophilicized-modified carbodiimide compound (C) is excellent in waterdispersibility, so that it is possible to improve the storage stabilityof the aqueous intermediate coating composition by forming theabove-mentioned curing agent composition even when the dispersibility inwater of the polyisocyanate compound (B) is not sufficient.

Generally, the resin solid concentration of the aqueous intermediatecoating composition may preferably be set to 15 to 60% by mass though itvaries depending on the application condition.

Aqueous Base Coating Composition

The aqueous base coating composition to be used in the present inventioncomprises an aqueous resin having a hydroxyl group and a carboxyl group(A2), a melamine resin (D), a weak acid catalyst (E), and an aqueouspolyurethane resin (F).

Aqueous Resin Having a Hydroxyl Group and a Carboxyl Group (A2)

The aqueous resin (A2) contained in the aqueous base coating compositionis the same type of resin as the above-mentioned aqueous resin (A1)contained in the aqueous intermediate coating composition, but is aresin without the provision regarding the range of the acid value in theaqueous resin (A1). That is, the aqueous resin (A2) contained in theaqueous base coating composition is a resin having a hydroxyl value of80 to 200 mgKOH/g in terms of resin solid content. The aqueous resin(A2) contained in the aqueous base coating composition may preferablyhave an acid value of 10 to 40 mgKOH/g.

The fact that the aqueous resin (A2) contained in the aqueous basecoating composition has a hydroxyl value of 80 to 200 mgKOH/g in termsof resin solid content offers an advantage that the crosslinking densityof a resulting coating film falls into a good range while keeping thecoating material stability good in the aqueous base coating compositioncontaining the above-mentioned components, so that the performance suchas water resistance is improved. This is considered to be because thelow-temperature curability of the coating composition is secured due tothe fact that the hydroxyl value of the aqueous resin (A2) is relativelyhigh like the above-mentioned range and the melamine resin (D) describedbelow, the weak acid catalyst (E), and the aqueous polyurethane resin(F) are further contained in the aqueous base coating composition, andthus the crosslinking density in a resulting coating film will fallwithin a good range.

The aqueous resin (A2) may preferably be contained within the range of20 to 60% by mass based on the resin solid content of the aqueous basecoating composition, for example. The above range may more preferably befrom 30 to 50 mass%.

Melamine Resin (D)

The melamine resin (D) contained in the aqueous base coating compositionof the present invention is has a structure in which R⁵ to R¹⁰ groupsare bonded to a melamine nucleus (triazine nucleus) via three nitrogenatoms, as represented in the following formula (1). The melamine resinis generally composed of a polynuclear body in which a plurality ofmelamine nuclei are bonded to each other. On the other hand, themelamine resin may have a mononuclear body which is composed of onemelamine nucleus. Furthermore, a structure of melamine nucleusconstituting the melamine resin may preferably be represented by thefollowing formula (1).

In the above formula (1), each R⁵ to R¹⁰ is same or different, andrepresents a hydrogen atom (imino group), CH₂OH (methylol group),CH₂OR¹¹, or a bonding portion with another melamine nucleus. R¹¹ is analkyl group, preferably an alkyl group having 1 to 4 carbon atoms suchas a methyl group, an ethyl group, a propyl group, a butyl group or thelike.

In the present invention, the melamine resin has an average imino groupamount of 1.0 or more and an average methylol group amount of 0.5 ormore, per one melamine nucleus. That is, all of R⁵ to R¹⁰ include anaverage of 1.0 or more imino group(s), and include an average of 0.5 ormore methylol group(s). Such a melamine resin is an iminomethylol typemelamine resin derivative having both of an imino group and a methylolgroup in one molecule. The melamine resin can be self-condensed by theimino group. The methylol group can react with a hydroxyl group of theaqueous resin to cause co-condensation. A crosslinked structure isformed by a reaction of the melamine resin with the aqueous resin (A2),and a coating film having good physical properties and quality can beobtained.

In the present invention, by setting amounts (average values) of iminogroup and methylol group per one melamine nucleus within the abovespecified range, both low temperature curability and storage stabilitycan be improved. A preferable lower limit value of the average iminogroup amount may be 1.2. An upper limit value of the average imino groupamount is not particularly limited, but from the viewpoint ofproduction, the preferable upper limit value may be 3.0. A preferablelower limit value of the average methylol group amount may be 0.65, anda more preferable lower limit value may be 0.7. An upper limit value ofthe average amount of methylol groups is not particularly limited, butfrom the viewpoint of production, the preferable upper limit value maybe 1.0.

A number average molecular weight measured by GPC in the melamine resinmay preferably be 300 to 1,300. When the number average molecular weightis within the above range, film appearance, alkali resistance and waterresistance of the coating film can be improved. A more preferable rangeof the number average molecular weight may be 300 to 1,000, and aparticularly preferable range may be 300 to 800.

The above-mentioned melamine resin can be synthesized by adjusting theaverage imino group amount and average methylol group amount per onemelamine nucleus so as to have a high value as described above by amanufacturing method commonly used by those skilled in the art. As themelamine resin, a commercially available product may be used. Specificexamples of commercially available products include “CYMSL (registeredtrademark) 701”, “CYMEL 202” manufactured by ALLNEX Japan Corporation.In the examples described below, in addition to these commerciallyavailable products, those prepared so that the average imino groupamount and the average methylol group amount are higher than these areused. The above melamine resins may be used alone, or two or more ofthem may be used in combination.

Weak Acid Catalyst (E)

The weak acid catalyst (E) may be an acid having a relatively low degreeof ionization in an aqueous solution. For example, an acid catalysthaving pKa (H₂O) of more than 1 may be suitable. pKa (H₂O) is an aciddissociation constant for water, and a generally known value at 20° C.may be used. Examples of such weak acid catalysts include carboxylicacids such as acetic acid, propionic acid and benzoic acid; and,phosphoric acid, phosphoric acid esters, phenol, carbonic acid, boricacid, hydrogen sulfide and the like. As the weak acid catalyst (E), anyone of these may be used, or two or more of them may be used incombination. It may be particularly preferable that the weak acidcatalyst (E) contains a phosphate compound. By using a weak acidcatalyst, storage stability can be secured while improving lowtemperature curability.

As the weak acid catalyst (E), a commercially available acid catalystcan be used. An example of the commercially available products mayinclude, for example, CYCAT series, those having pKa (H₂O) of more than1.

The aqueous coating composition does not substantially include an acidcatalyst having pKa (H₂O) of 1 or less. The team “substantially” meansthat an amount of the acid catalyst having pKa (H₂O) of 1 or less basedon the aqueous coating composition does not exceed 0.01% by mass. If theacid catalyst having pKa (H₂O) of 1 or less is contained beyond theabove concentration, the effect of low temperature curing cannot beobtained.

Aqueous Polyurethane Resin (F)

The aqueous base coating composition comprises an aqueous polyurethaneresin (F) in addition to the above components. Due to the inclusion ofthe specific aqueous polyurethane resin (F) in the aqueous base coatingcomposition, even in the case where the aqueous intermediate coatingcomposition and the aqueous base coating composition are applied andthen the aqueous coating composition is baked and cured underlow-temperature curing conditions, it is possible to farm a strongcoating film through fusion of the aqueous polyurethane resin withitself and other components, and thus, a multilayer coating filmexcellent in adhesion between coating films and water-resistant adhesionis to be obtained.

The aqueous polyurethane resin (F) is a polymer obtained by using apolyol compound (F-1), a compound having an active hydrogen group and ahydrophilic group in the molecule (F-2), an organic polyisocyanate(F-3), and, if necessary, a chain extender and a polymerizationterminator, and can be prepared by dissolving or dispersing a resultingpolymer in water.

The polyol compound (F-1) is not particularly limited as long as it is apolyol compound having two or more hydroxyl groups. Examples of thepolyol compound (F-1) include polyhydric alcohols such as ethyleneglycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol,trimethylolpropane, and glycerol; polyether polyols such as polyethyleneglycol, polypropylene glycol, and polytetramethylene ether glycol;polyester polyols obtained from a dicarboxylic acid such as adipic acid,sebacic acid, itaconic acid, maleic anhydride, phthalic acid, andisophthalic acid, and a glycol such as ethylene glycol, triethyleneglycol, propylene glycol, butylene glycol, tripropylene glycol, andneopentyl glycol; polycaprolactone polyol; polybutadiene polyol;polycarbonate polyol; and polythioether polyol. The polyol compound(F-1) may be used singly, or two or more species thereof may be used incombination. The polyol compound (F-1) may preferably have anumber-average molecular weight of 500 to 5000.

Examples of the compound having an active hydrogen group and ahydrophilic group in the molecule (F-2) include compounds known ascompounds containing active hydrogen and an anionic group {an anionicgroup or an anion-forming group (a group that reacts with a base to forman anionic group and, in this case, that is converted into an anionicgroup by neutralizing with a base before, during or after aurethanization reaction)} (those disclosed in JP-B-42-24192 andJP-B-55-41607; specific examples include dimethylolalkanoic acids suchas α, α-dimethylolpropionic acid, α, α-dimethylolbutyric acid, anddimethylolacetic acid), compounds known as compounds having activehydrogen and a cationic group in the molecule (e.g., those disclosed inJP-B-43-9076), and compounds known as compounds having active hydrogenand a nonionic group (e.g., those disclosed in JP-B-48-41718;specifically, polyethylene glycol and alkylalcohol alkylene oxideadducts, etc.). It may be preferred to use a dimethylolalkanoic acid asthe compound having an active hydrogen group and a hydrophilic group inthe molecule (F-2).

The organic polyisocyanate (F-3) is not particularly limited as long asit has two or more isocyanate groups in the molecule. Specific examplesof the organic polyisocyanate (F-3) include:

-   aliphatic diisocyanates having 2 to 12 carbon atoms such as    hexamethylene diisocyanate, 2,2,4-trimethylhexane diisocyanate, and    lysine diisocyanate;-   alicyclic diisocyanates having 4 to 18 carbon atoms such as    1,4-cyclohexane diisocyanate, isophorone diisocyanate,    4,4′-dicyclohexylmethane diisocyanate, methylcyclohexylene    diisocyanate, and isopropylidenecyclohexyl-4,4′-diisocyanate;-   aromatic diisocyanates such as 2,4-toluylene diisocyanate,    2,6-toluylene diisocyanate, diphenylmethane-4,4′-diisocyanate,    1,5′-naphthene diisocyanate, tolidine diisocyanate,    diphenylmethylmethane diisocyanate, tetraalkyldiphenylmethane    diisocyanate, xylylene diisocyanate, tetramethylxylylene    diisocyanate, 4,4′-dibenzyl diisocyanate, and 1,3-phenylene    diisocyanate; and-   triisocyanates such as lysine ester triisocyanate, triphenylmethane    triisocyanate, 1,6,11-undecane triisocyanate,    1,8-diisocyanate-4,4-isocyanatemethyloctane, 1,3,6-hexamethylene    triisocyanate, and bicycloheptane triisocyanate.

These polyisocyanates may be used in the form of dimer or trimer thereof(isocyanurate linkage), and may be reacted with an amine and used as abiuret. Moreover, it is also possible to use polyisocyanates having aurethane linkage resulting from reaction of such polyisocyanatecompounds with polyols.

It may be more preferred to use an aliphatic diisocyanate as the organicpolyisocyanate (F-3). Preparation of the aqueous polyurethane resin (F)using an aliphatic diisocyanate offers an advantage that the waterpermeability of a resulting coating film can be adjusted to a properrange and good low-temperature initial water resistance can be obtained.

The chain extender that can be used if necessary in the preparation ofthe aqueous polyurethane resin (F) is not particularly limited as longas it has two or more active hydrogen groups, and examples thereofinclude low molecular weight polyols (number-average molecular weight ofless than 500) and polyamines. Examples of the low molecular weightpolyols include ethylene glycol, propylene glycol, 1,4-butanediol,3-methylpentanediol, 2-ethyl-1,3-hexanediol, and trimethylolpropane.Examples of the polyamines include ethylenediamine,hexamethylenediamine, diethylenetriamine, hydrazine, xylylenediamine,and isophoronediamine.

Examples of the polymerization terminator include a compound having oneactive hydrogen in its molecule, and a monoisocyanate compound.

Examples of the compound having one active hydrogen in its moleculeinclude monoalcohols (e.g., alkyl alcohols such as methanol, butanol andoctanol, alkyl alcohol alkylene oxide adducts), and monoamines (e.g.,alkylamines such as butylamine and dibutylamine).

Examples of the monoisocyanate compound include methylisocyanate,ethylisocyanate, propylisocyanate, butylisocyanate, laurylisocyanate,cyclohexylisocyanate, phenylisocyanate, and tolyleneisocyanate.

A reaction method in producing the aqueous polyurethane resin (F) may beany method of a one-shot method in which the respective components arereacted at once and a multistage method in which the respectivecomponents are reacted in steps {a method of producing the resin byreacting part of an active hydrogen-containing compound (e.g.,macromolecular polyol) with a polyisocyanate, thereby forming an NCOterminated prepolymer, and then reacting the remainder of the activehydrogen-containing compound}. The reaction of synthesizing the aqueouspolyurethane resin (F) is performed usually at 40 to 140° C., preferably60 to 120° C. In order to accelerate the reaction, there may be used acatalyst that is usually used for a urethanization reaction, such as atin-based catalyst including dibutyltin laurate and tin octylate or anamine-based catalyst including triethylenediamine. In addition, theabove reaction may be carried out in an organic solvent that is inert toisocyanate (e.g., acetone, toluene, dimethylformamide, etc.), and thesolvent may be added either during the reaction or after the reaction.

The aqueous polyurethane resin (F) in the present invention can beprepared by treating a resulting polymer with a known method (a methodof forming an anionic group by neutralization with a base in the case ofan anion-forming group, a method of forming a cationic group with aquaternarizing agent or a method of forming a cationic group byneutralization with an acid in the case of a cation-forming group) andthen dispersing the polymer in water.

The step of dissolving the polymer in water is not particularly limited,and it may be performed either after the reaction or at a stage duringthe course of the multistage method. For example, when dissolving thepolymer in water at the stage of an NCO terminated prepolymer, theaqueous polyurethane resin (F) is obtained by dissolving the polymer inwater while extending the chain with water and/or a polyamine.

When using an organic solvent inert to the isocyanate, solvent removalmay be carried out after dissolving the polymer in water.

The aqueous polyurethane resin (F) in the present invention is requiredto have a glass transition point (Tg) of −50° C. or less and a curedfilm of the aqueous polyurethane resin (F) is required to have anelongation at break of 400% or more at −20° C.

When the glass transition point (Tg) of the aqueous polyurethane resin(F) exceeds −50° C., a resulting multilayer coating film will be poor incoating film adhesion, chipping resistance and water resistance. Theglass transition point (Tg) may more preferably be −55° C. or less, andeven more preferably −58° C. or less. The glass transition point (Tg) ofthe aqueous polyurethane resin (F) can be measured using a differentialscanning calorimeter.

When the cured film of the aqueous polyurethane resin (F) has anelongation at break of less than 400% at −20° C., a resulting multilayercoating film will be poor in coating film adhesion, chipping resistanceand water resistance. The elongation at break may more preferably be500% or more.

The elongation at break of the cured film of the aqueous polyurethaneresin (F) can be determined in accordance with JIS K7127.

Specifically, 95 parts by mass (resin solid content amount) of theaqueous polyurethane resin (F) and 5 parts by mass (resin solid contentamount) of the hydrophilicized-modified carbodiimide compound (C) aremixed. The resulting mixture is applied uniformly with a doctor bladesuch that the dry film thickness is 20 μm. After leaving at rest at 20°C. for 10 minutes, the resulting mixture is preheated at 80° C. for 3minutes, thereby volatilizing water. Then, the resulting mixture isbaked at 120° C. for 30 minutes, and thus a cured film is prepared. Theresulting cured film is subjected to a tensile performance test at atesting temperature of −20° C. in accordance with JIS K7127 and anelongation ratio at the time of breaking is measured. The obtainedelongation ratio is taken as an elongation at break.

By mixing the aqueous polyurethane resin and the carbodiimide compoundrepresented by the formula (I), (II) or (III) and baking them to beformed into a film shape as described above, crosslinking and/or fusionbetween the aqueous polyurethane resin and the carbodiimide resinproceeds, and it becomes possible to evaluate the elongation at break ofthe aqueous polyurethane resin.

As the aqueous polyurethane resin (F), a commercially available productmay be used. Examples of the commercially available product include NeoRez Series, which are aqueous polyurethane resins available fromKusumoto Chemicals, Ltd., HUX Series, which are aqueous polyurethaneresins available from ADEKA Corporation, and UCOAT Series, PERMARINSeries, and U-Prene Series, which are aqueous polyurethane resinsavailable from Sanyo Chemical Industries, Ltd.

A content of the aqueous polyurethane resin (F) may preferably be 8% bymass or more, more preferably 10% by mass or more, and more preferably15% by mass or more based on the resin solid content of the aqueous basecoating composition. The fact that the content of the aqueouspolyurethane resin (F) is 8% by mass or more offers an advantage thateven in the case where the aqueous coating composition is baked andcured under low-temperature curing conditions, it is possible to form astrong coating film through fusion of the aqueous polyurethane resinwith itself and other components, and thus, a multilayer coating filmexcellent in adhesion between coating films and water-resistant adhesionis to be obtained. The upper limit of the content may preferably be 50%by mass or less, and more preferably 30% by mass or less.

Other Resins

The aqueous base coating composition may contain a resin component otherthan the aqueous resin (A2) (other resin), if necessary. One example ofsuch other resin includes a resin that is prepared in the same manner asthe aqueous resin (A2) and that has a hydroxyl value of less than 80mgKOH/g. Examples of such other resin include resins having a hydroxylgroup, such as polyether diol and polycarbonate diol.

Such other resin can be used in an arbitrary amount, provided thatfunctions (water resistance, chipping resistance, etc.) of the aqueousbase coating composition are not damaged. The resin having a hydroxylvalue of less than 80 mgKOH/g may preferably be contained, for example,in the range of 15 to 45% by mass based on the resin solid content ofthe aqueous base coating composition.

Preparation of Aqueous Base Coating Composition

In the aqueous base coating composition, a mass ratio of the aqueousresin (A2) and the melamine resin (D) is in the range of (A2)/(D)=0.7 to3 in terms of solid content. When an amount of the aqueous resin (A2)exceeds the above range and an amount of the melamine resin (D) is lessthan the above range, curing (crosslinking) reactivity may be affectedand low temperature curability may not be sufficient. On the other hand,when an amount of the aqueous resin (A2) is decreased and an amount ofthe melamine resin (D) is increased, an amount of acid group in theaqueous base coating composition increases, so that curing(crosslinking) reactivity in storage may be increased and storagestability may be deteriorated. Preferably, the mass ratio of the aqueousresin (A2) to the melamine resin (D) may preferably be in the range of(A2)/(D)=0.7 to 2.7 in terms of solid content, more preferably withinthe range of 0.8 to 2.7.

An amount of the weak acid catalyst (E) in the aqueous base coatingcomposition is within the range of 0.1 to 10.0 parts by mass, based on100 parts by mass of solid contents ((A2)+(D)) of the aqueous resin (A2)and the melamine resin (D) contained in the aqueous base coatingcomposition. In the case of using a weak acid catalyst dissolved ordispersed in a solvent, an amount of the weak acid catalyst (E) iscalculated based on an amount of the active ingredient excluding thesolvent (volatile component). When an amount of the weak acid catalystexceeds the above range and exceeds the upper limit value, storagestability may decrease or the physical properties of the coating filmmay deteriorate (e.g., contraction in appearance of the coating film mayappear). When an amount of the weak acid catalyst is less than the lowerlimit in the above range, curing (crosslinking) reactivity may belowered, low temperature curability may not be sufficient, and physicalproperties of the coating film may be deteriorated. An amount of theweak acid catalyst (E) in the aqueous base coating composition may morepreferably be within the range of 0.1 to 5.0 parts by mass, based on 100parts by mass of solid contents ((A2)+(D)) of the aqueous resin (A2) andthe melamine resin (D) in the aqueous base coating composition. Theabove ranges may be more preferable when the weak acid catalyst (E)contains a phosphate compound.

In the aqueous base coating composition, a neutralization ratio to thecoating composition may preferably be 50% or more. That is, in case thatthe total amount of acid groups contained in the coating composition is100 mol %, it is preferable that the coating composition is neutralizedwith a basic compound so as to have a base amount of 50 mol % or morebased on the total amount of the acid groups. The above state means thatan amount of acid group without counter base (i.e., free acid group)theoretically exists not more than 50% in the coating composition. Whena neutralization ratio is within the above range, stability of thecoating composition in the aqueous medium can be maintained, and storagestability can be sufficiently secured. When a base amount is less than50%, curing (crosslinking) reaction between the aqueous resin and themelamine resin during storage may be accelerated, thereby possiblyreducing the storage stability. The neutralization ratio may preferablybe more than 50%. In addition, the upper limit of the neutralizationratio to the coating composition may preferably be 150% or less. When itexceeds 150%, curing (crosslinking) reactivity may be lowered, lowtemperature curability may not be sufficient and the physical propertiesof the coating film may be deteriorated, because action of weak acidcatalyst (E) is hindered.

As the basic compound, those commonly used as a neutralizing agent canbe used. Examples of the basic compound include ammonia; amine compoundssuch as methylamine, ethylamine, dimethylamine, diethylamine,trimethylamine, triethylamine, dimethylethanolamine, diethanolamine,diethylaminoethanol, triethanolamine; hydroxides and carbonates ofalkali metals or alkaline earth metals, and the like. The above compoundmay be used alone, or two or more of them may be used in combination.

In the aqueous base coating composition, since a melamine resin having alarge amount of imino group and methylol group is used, settings of therange of the mass ratio of the aqueous resin (A2) and the melamine resin(D) and the range of the neutralization ratio will affect storagestability. In a preferred embodiment of the present invention, whenthese are set in the optimum range, storage stability can besufficiently secured even in the low-temperature curing technique, andthis is one of the technical significance of the present invention.

The aqueous base coating composition may contain other than the abovecomponents, which, for example, the above-mentionedhydrophilicized-modified carbodiimide compound, water-dispersed blockisocyanate. Containing at least one of them can improve low temperaturecurability.

The aqueous base coating composition can be prepared by mixing therespective components to constitute the coating composition with a meansthat is usually used. If necessary, the aqueous base coating compositionmay contain a pigment, a surface conditioner (a defoaming agent, aleveling agent, etc.), a pigment dispersing agent, a plasticizer, afilm-forming assistant, an ultraviolet absorber, an antioxidant, a flameretardant, an antistatic agent, an electrostatic auxiliary, a heatstabilizer, a light stabilizer, a solvent (water, organic solvent), andother additives.

When the aqueous base coating composition contains a pigment, a contentof the pigment may be set within a range usually set according to anapplication. For example, the PWC (Pigment Weight Concentration) of thepigment in % by mass based on 100 parts by mass in total of the totalsolid contents of the resin and the curing agent in the aqueous basecoating composition may preferably be adjusted to 0.1 to 50% by mass.

Method for Forming a Multilayer Coating Film

The method for forming a multilayer coating film of the presentinvention includes:

an intermediate coating film formation step of applying an aqueousintermediate coating composition to an object to be coated to foam anuncured intermediate coating film;

a base coating film formation step of applying an aqueous base coatingcomposition onto a resulting uncured intermediate coating film to forman uncured base coating film; and

a curing step of curing the resulting uncured intermediate coating filmand the base coating film by heating.

In the method for forming a multilayer coating film of the presentinvention, use of the specific aqueous intermediate coating compositionand the aqueous base coating composition described above makes itpossible to obtain a coating film excellent in coating film physicalproperties even under low-temperature curing conditions. The heat curingtemperature of the coating film in the above curing step is notparticularly limited, and may preferably be 70 to 120° C., morepreferably 70 to 110° C., and even more preferably 70 to 100° C. In themethod for forming a multilayer coating film of the present inventioncan be carried out under such a low-temperature curing condition of 100°C. or less. Such a low-temperature curing condition may be a curingcondition where the heat curing temperature of the coating film is 70 to90° C.

Object to be Coated

Examples of the object to be coated in the above method include steelplates of metal such as iron, steel, stainless steel, aluminum, copper,zinc, and tin and alloys thereof; resins such as polyethylene resin, EVAresin, polyolefin resins (polyethylene resin, polypropylene resin,etc.), vinyl chloride resin, styrol resin, polyester resins (includingPET resin, PBT resin, etc.), polycarbonate resin, acrylic resin,acrylonitrile-butadiene-styrene (ABS) resin, acrylonitrile-styrene (AS)resin, polyamide resin, acetal resin, phenol resin, fluororesin,melamine resin, urethane resin, epoxy resin, and polyphenylene oxide(PPO); and organic-inorganic hybrid materials. These may have beenmolded.

The steel plate may be in a state where an electrodeposition coatingfilm is formed after being subjected to a chemical conversion treatment.Examples of the chemical conversion treatment include zinc phosphateconversion, zirconium conversion, and chromic acid conversion. Examplesof the electrodeposition coating film include electrodeposition coatingfilms obtained by electrodeposition using a cationic electrodepositioncoating composition or an anionic electrodeposition coating composition.

The resin may, if necessary, have been subjected to vapor cleaning usingan organic solvent or may have been subjected to cleaning using aneutral detergent. Moreover, the resin may have been subjected to primercoating according to necessity.

The method for forming a multilayer coating film of the presentinvention is characterized in that it can provide a coating filmexcellent in coating film properties even under low-temperature curingconditions. Therefore, the object to be coated for which the method ofthe present invention can be used suitably may be, for example, anobject to be coated including a steel plate part and a resin part. Bythe method for forming a multilayer coating film of the presentinvention, a multilayer coating film is formed on the object to becoated, so that it becomes possible to form a multilayer coating filmhaving good physical properties at both the resin part and the steelplate part without applying heat by which thermal distortion of theresin part will be caused. By use of the method for forming a multilayercoating film of the present invention, a common coating composition canbe applied even to different materials, namely, resin and steel plate.This offers an advantage that the hues of the coating films to be formedcan be matched at a higher level.

Examples of other object to be coated that is suitable as the object tobe coated in the method for forming a multilayer coating film of thepresent invention include industrial machines and construction machines.Industrial machines and construction machines are generally large andhave a feature that their constituting base materials (steel plates) arethick as compared with automobile bodies in order to withstand a largeload. Therefore, when such an industrial machine or a constructionmachine is the object to be coated, the object to be coated is large inheat capacity and there is a problem that heat is not transferredsufficiently to the object to be coated in a heating oven. The methodfor forming a multilayer coating film of the present invention ischaracterized in that the aqueous coating composition can be cured atlow temperatures and in that a coating film having a high crosslinkingdensity can be obtained even when the aqueous coating composition iscured at low temperatures. Therefore, the method for forming amultilayer coating film of the present invention can be used suitablyalso for application to objects to be coated which are large in heatcapacity and difficult to be subjected to high-temperature heat curingtreatment after application, namely, industrial machines andconstruction machines.

The aqueous intermediate coating composition and the aqueous basecoating composition can be applied by an application method usuallyused. For example, when the aqueous intermediate coating composition andthe aqueous base coating composition are applied to an automobile body,they can be applied by multi-stage application, preferably two-stageapplication with use of air-electrostatic spray, or alternatively, therecan be used an application method combining air electrostatic spray anda rotary atomization type electrostatic applicator, which is so-called“μμ (micro micro) bell”, “μ (micro) bell”, “metallic bell” or the like,in order to improve the appearance of a resulting coating film.

The thickness of the coating film of the aqueous intermediate coatingcomposition may be chosen appropriately according to the desired use.The film thickness may preferably be, for example, 8 to 40 μm in termsof a dry film thickness, and more preferably 15 to 30 μm.

The thickness of the coating film of the aqueous base coatingcomposition can be chosen appropriately according to the desired use.The film thickness may preferably be, for example, 10 to 30 μm in termsof a dried film thickness.

The method for forming a coating film of the present invention alsoincludes an embodiment in which in a state where the base coating filmis still uncured, a clear coating composition is further applied,thereby forming a clear coating film, and then an uncured multilayercoating film is cured. This method can omit a baking drying oven andtherefore may be preferable from economical viewpoint and environmentalprotection viewpoint.

Examples of the clear coating composition that can be used suitably inthe above coating step include a urethane clear coating composition.Examples of the urethane clear coating composition include clear coatingcompositions containing a hydroxyl group-containing resin and anisocyanate compound curing agent. The isocyanate compound as a curingagent is not particularly limited, and examples thereof includealiphatic isocyanates such as trimethylene diisocyanate, tetramethylenediisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate(HDI), and trimethylhexamethylene diisocyanate; aliphatic cyclicisocyanates such as 1,3-cyclopentane diisocyanate, 1,4-cyclohexanediisocyanate, and 1,2-cyclohexane diisocyanate; aromatic isocyanatessuch as xylylene diisocyanate (XDI), 2,4-tolylene diisocyanate (TDI),and 2,6-tolylene diisocyanate; alicyclic isocyanates such as isophoronediisocyanate (IPDI) and norboriiane diisocyanate; multimers such asbiuret type and nurate type of these isocyanates; and mixtures thereof.

The hydroxyl value of the hydroxyl group-containing resin may preferablybe within a range of 20 to 200 mgKOH/g. When the hydroxyl value exceedsthe upper limit, the water resistance of a coating film willdeteriorate, and when the hydroxyl value is less than the lower limit,the curability of a coating film will deteriorate. The lower limit maymore preferably be 30 mgKOH/g, and the upper limit may more preferablybe 180 mgKOH/g.

The number-average molecular weight of the hydroxyl group-containingresin may preferably be within a range of 1000 to 20000. When thenumber-average molecular weight is less than 1000, the workability andthe curability may be insufficient. When the number-average molecularweight exceeds 20000, a nonvolatile portion during coating will bedecreased and the workability may deteriorate. The lower limit may morepreferably be 2000 and the upper limit may more preferably be 15000.

Moreover, the hydroxyl group-containing resin may preferably have anacid value within a range of 2 to 30 mgKOH/g. When the acid valueexceeds the upper limit, the water resistance of a coating film willdeteriorate, and when the acid value is less than the lower limit, thecurability of a coating film will deteriorate. The lower limit may morepreferably be 3 mgKOH/g, and the upper limit may more preferably be 25mgKOH/g.

A content of the isocyanate compound relative to the hydroxylgroup-containing resin may be chosen suitably within a range usuallyused by those skilled in the art. For example, it may be preferable touse the isocyanate compound in such an amount that an equivalent ratioof isocyanate groups (NCO) to hydroxyl groups (OH) (NCO/OH) falls withinthe range of 0.5 to 1.7. The lower limit may more preferably be 0.7, andthe upper limit may more preferably be 1.5.

A method for producing the clear coating composition is not particularlylimited and a method well-known to those skilled in the art may be used.The clear coating composition to be used may be a commercially availableproduct. Examples of the cumercially available product include PolyureExcel O-1100 Clear and O-1200 Clear (produced by Nippon Paint AutomotiveCoatings Co., Ltd., isocyanate-curing type clear coating compositions).

When using the clear coating composition, a multilayer coating film canbe formed by applying the aqueous base coating composition, therebyforming an uncured base coating film, and then applying the clearcoating composition by wet-on-wet, and subsequently baking and curingthem, for example, at 70 to 120° C., preferably at 70 to 110° C., morepreferably at 70 to 100° C. for 10 to 30 minutes. A furtherlow-temperature curing condition may be a curing condition to performbaking and curing at 70 to 90° C. for 10 to 30 minutes.

In the present invention, according to the material of the object to becoated, a clear coating composition other than the above-mentionedurethane clear coating composition can be used. For example, an acidepoxy curable type clear coating composition, an acrylic-melaminecurable type clear coating composition, etc. may be used. Examples ofsuch clear coating compositions include “Macflow O-570 Clear” and“Macflow O-1820 Clear” available from Nippon Paint Automotive CoatingsCo., Ltd., which are clear coating compositions containing a polyepoxideand a polyacid, and “Super rack O-100 Clear” (trade name) available fromNippon Paint Automotive Coatings Co., Ltd., which is a clear coatingcomposition containing an acrylic resin and melamine curing agent. Theheat curing conditions for the case of using such clear coatingcompositions may be chosen appropriately according to the composition ofthe respective clear coating compositions. One example of the heatcuring conditions for the case of using such clear coating compositionsincludes a condition of heating at 120 to 140° C. for 10 to 30 minutes.

As a method for applying the clear coating composition, theabove-described known coating method can be used, and for example, thecomposition can be applied with an air spray, electrodeposition, or thelike. The clear coating composition may preferably be applied so thatthe dry film thickness is generally 10 to 80 μm, preferably 20 to 50 μm.

EXAMPLES

The present invention will be described more specifically with referenceto the following examples, but the present invention is not limited tothe examples. In the examples, “parts” and “%” are on a mass basisunless otherwise indicated.

Production Example 1 Production of Acrylic Emulsion having HydroxylGroup and Carboxyl Group (AcEm-1)

A reaction vessel equipped with a stirrer, a nitrogen inlet tube, atemperature controller, a condenser, and a dropping funnel was chargedwith 2,000 parts of deionized water, and was then heated to 80° C. withstirring under nitrogen atmosphere.

A pre-emulsion was prepared by adding a monomer of 103 parts of styrene,290 parts of n-butyl methacrylate, 280 parts of n-butyl acrylate, 302parts of hydroxyethyl acrylate, 26 parts of acrylic acid, and, 3 partsof dodecyl mercaptan, 100 parts of LATEMUL PD-104 (produced by KaoCoLporation. 20% aqueous solution) as an emulsifier, to 1,000 parts ofdeionized water. Then, the resultant was emulsified, and resultingpre-emulsion was dropped over 2 hours together with an aqueous initiatorsolution prepared by dissolving 3 parts of ammonium persulfate in 300parts of deionized water.

After the completion of the dropping, the reaction was continued at 80°C. for 1 hour, followed by cooling, and 8.2 parts ofN,N-dimethylaminoethanol was added and thus an acrylic emulsion having aresin solid content of 30% by mass was obtained. The hydroxyl value ofthe acrylic emulsion, in terms of resin solid content, calculated fromthe monomer composition was 130 mgKOH/g and the acid value was 20mgKOH/g. The acrylic resin in the resulting acrylic emulsion had anumber-average molecular weight of 45,000 as determined by GPCmeasurement after removing water.

Production Example 2 Production of Aqueous Polyester Dispersion havingHydroxyl Group and Carboxyl Group (PE-DP)

A reaction vessel equipped with a stirrer, a nitrogen inlet tube, atemperature controller, a condenser, and a decanter was charged with 250parts of trimethylolpropane, 824 parts of adipic acid, and 635 parts ofcyclohexanedicarboxylic acid, then the mixture was heated to 180° C.,and then a condensation reaction was carried out until no more waterdistilled out. After cooling to 60° C., 120 parts of phthalic anhydridewas added and the mixture was heated to 140° C. and held for 60 minutes,and thus a polyester resin having a number-average molecular weight of2,000 as determined by GPC measurement was obtained. Fifty nine parts ofdimethylaminoethanol (corresponding to 80% of the acid value of theresin (neutralization ratio: 80%)) was added at 80° C., and 1920 partsof deionized water was further added, followed by stirring, and thus anaqueous polyester dispersion having a resin solid content of 45% by masswas obtained. The hydroxyl value of the aqueous polyester dispersion interms of resin solid content was 90 mgKOH/g and the acid value was 35mgKOH/g.

Production Example 3 Production of Hydrophilicized Modified CarbodiimideCompound (1)

By reacting 700 parts of 4,4-dicyclohexylmethane diisocyanate with 7parts of 3-methyl 1-phenyl 2-phospholene-1-oxide at 170° C. for 7 hours,obtained was a carbodiimide compound with the structure represented bythe above formula (a), the carbodiimide compound having threecarbodiimide groups in one molecule and having isocyanate groups at itsboth ends.

Next, to 180 parts of the produced 4,4-dicyclohexylmethanecarbodiimidehaving isocyanate ends were added 95 parts of PTMG-1000(polytetramethylene glycol having a number-average molecular weight of1,000 produced by Mitsubishi Chemical; repeat number of tetramethyleneoxide calculated from number-average molecular weight was 13.6) and 0.2parts of dibutyltin dilaurate, and the mixture was then heated to 85° C.and held for 2 hours.

Subsequently, 86.4 parts of Methyl Poly Glycol 130 (polyethylene glycolmonomethyl ether produced by Nippon Nyukazai Co., Ltd.; repeat number ofethylene oxide calculated from hydroxyl value of 130 mgKOH/g was 9) wasadded and then the mixture was held at 85° C. for 3 hours. Afterconfirming disappearance of a peak of NCO by IR measurement, thereaction was finished, followed by cooling to 60° C., and then deionizedwater was added, and thus an aqueous dispersion of ahydrophilicized-modified carbodiimide compound (1) having a resin solidcontent of 40% by mass was obtained. The resultinghydrophilicized-modified carbodiimide compound was a compoundrepresented by the above formula (I).

Production Example 4 Production of Hydrophilicized Modified CarbodiimideCompound (2)

To 90 parts of the 4,4-dicyclohexylmethanecarbodiimide having isocyanateends produced in Production Example 3 were added 120 parts ofpolypropylene glycol monobutyl ether having an average repeat number of19, 43.2 parts of Methyl Poly Glycol 130, and 0.07 parts of dibutyltindilaurate, and the temperature was held at 80° C. until absorption ofNCO disappeared in an IR spectrum. After cooling to 60° C., deionizedwater was added and thus an aqueous dispersion of ahydrophilicized-modified carbodiimide compound (2) having a resin solidcontent of 25% was obtained. The resulting hydrophilicized-modifiedcarbodiimide compound was a compound represented by the above formula(III).

In the resulting hydrophilicized-modified carbodiimide compound, theratio of (i) a structure resulting from elimination of a hydroxyl groupfrom polyethylene glycol monoalkyl ether and (ii) a structure resultingfrom elimination of a hydroxyl group from polypropylene glycol monoalkylether was (i):(ii)=1.0:1.0.

Production Example 5 Production of Hydrophilicized Modified CarbodiimideCompound (3)

By reacting 393 parts of 4,4-dicyclohexylmethane diisocyanate with 8parts of 3-methyl-1-phenyl -2-phospholene-1-oxide at 180° C. for hours,obtained was a carbodiimide compound having four carbodiimide groups inone molecule and having isocyanate groups at both ends. Here, 130 partsof polyethylene glycol monomethyl ether having an oxyethylene grouprepeat number of 9 and 0.2 parts of dibutyltin dilaurate were added,followed by heating at 90° C. for 2 hours, and thus a carbodiimidecompound having an isocyanate group and a hydrophilic group at its endswas obtained. In addition, 300 parts of GP-3000 (trihydric polyol havinga structure in which 17 mol, in average, of propylene oxide was added torespective three hydroxyl groups of glycerol, produced by Sanyo ChemicalIndustries, Ltd.) was added and was reacted at 90° C. for 6 hours. Afterconfirming disappearance of a peak of NCO by IR measurement, thereaction was finished and thus a hydrophilicized-modified carbodiimidecompound (3) was obtained. Deionized water was added thereto and thus anaqueous dispersion of the hydrophilicized-modified carbodiimide compound(3) having a resin solid content of 30% by mass was obtained. Theresulting hydrophilicized-modified carbodiimide compound was a compoundrepresented by the above formula (II).

Production Example 6 Production of Coloring Pigment Paste

After preliminarily mixing 9.2 parts of a commercially availabledispersing agent “Disperbyk 190” (produced by BYK-Chemie), 17.8 parts ofion-exchanged water, and 73.0 parts of rutile type titanium dioxide, abead medium was added to the mixture in a paint conditioner, and mixedand dispersed at room temperature until the particle size reached 5 μmor less, and then the bead medium was removed by filtration and thus acoloring pigment paste was obtained.

Production Example 7 Production of Emulsion Resin (Aqueous Resin) havingHydroxyl Value of Less than 80 mgKOH/g

To reaction vessel containing 194.1 parts of ion-exchanged water wereadded 0.2 parts of ADEKA REASOAP NE-20(α-[1-[(allyloxy)methyl]-2-(nonylphenoxy)ethyl]-(1)-hydroxyoxyethylene,produced by ADEKA Corporation, aqueous solution having a solid contentof 80% by mass) and 0.2 parts of Aqualon HS-10 (polyoxyethylenealkylpropenylphenyl ether sulfate, produced by DES Co. Ltd.), themixture was then heated to 80° C. with mixing and stirring under anitrogen flow. Subsequently, a monomer mixture composed of 18.5 parts ofmethyl acrylate, 31.7 parts of ethyl acrylate, 5.8 parts of2-hydroxyethyl acrylate, 10.0 parts of styrene, 4.0 parts of acrylamide,0.3 parts of ADEKA REASOAP NE-20, 0.2 parts of Aqualon HS-10, and 70parts of ion-exchanged water as an α, β-ethylenically unsaturatedmonomer mixture for the first step, and an initiator solution composedof 0.2 parts of ammonium persulfate and 7 parts of ion-exchanged waterwere dropped in parallel into the reaction vessel over 2 hours. Afterthe completion of the dropping, aging was carried out at the sametemperature for 1 hour.

Further, a monomer mixture composed of 24.5 parts of ethyl acrylate, 2.5of 2-hydroxyethyl acrylate, 3.1 parts of methacrylic acid, 0.3 parts ofAqualon HS-10, and 30 parts of ion-exchanged water as an α,β-ethylenically unsaturated monomer mixture for the second step, and aninitiator solution composed of 0.1 parts of ammonium persulfate and 3parts of ion-exchanged water were dropped in parallel into the reactionvessel at 80° C. over 0.5 hours. After the completion of the dropping,aging was carried out at the same temperature for 2 hours.

Subsequently, the mixture was cooled to 40° C. and was filtered with a400 mesh filter. Further, a 10% by mass aqueous dimethylaminoethanolsolution was added and the pH was adjusted to 7, and thus an emulsionresin having an average particle diameter of 110 nm, a solid content of24% by mass, an acid value of 20 mgKOH/g and a hydroxyl value of 40mgKOH/g in terms of resin solid content was obtained. The glasstransition point was calculated to be 0° C. based on the whole monomercomposition.

Example 1 Production of Aqueous Intermediate Coating Composition

Stirred were 158 parts (resin solid content: 30% by mass) of the acrylicemulsion (AcEm-1) serving as an aqueous resin (A1) obtained inProduction Example 1, and 18.7 parts (resin solid content: 45% by mass)of the aqueous polyester dispersion (PE-DP) obtained in ProductionExample 4. To this was added 137.7 parts of the colored pigment paste ofProduction Example 6, the pH was adjusted to 8.0 with 0.01 parts ofdimethylethanolamine (produced by KISHIDA CHEMICAL Co., Ltd.), and 1.0part of ADEKA NOL UH-814N (trade name, urethane association typethickening agent, active ingredient: 30% by mass, produced by ADEKA Co.,Ltd.) was mixed and stirred, and the mixture was stirred until it becameuniform. To this was added 40.9 parts of BAYHYDUR 305 (polyisocyanatecompound having an ethylene oxide group produced by Sumika BayerUrethane Co., Ltd., ethylene oxide content: 20% by mass, isocyanategroup content: 16% by mass) serving as a polyisocyanate compound (B),and 8.3 parts (resin solid content: 40% by mass) of thehydrophilicized-modified carbodiimide compound (1) of PreparationExample 3 was further added while being stirred, and the mixture wasstirred, so that an aqueous intemediate coating composition wasobtained.

Production of Aqueous Base Coating Composition

Mixing of 116.7 parts (resin solid content: 30% by mass) of the hydroxylgroup-containing acrylic resin emulsion serving as an aqueous resin (A2)obtained in Production Example 1 and 104.2 parts (resin solid content:24% by mass) of the acrylic emulsion resin having a hydroxyl value ofless than 80 mgKOH/g obtained in Production Example 7 was performed. Tothe resulting mixture were added 66.7 parts (resin solid content: 30% bymass) of the aqueous polyurethane resin (F) given in the table, 21 parts(solid content: 65% by mass) of Alpaste MH8801 (aluminum pigmentproduced by Asahi Kasei Corporation) as a luster pigment, 5 parts of aphosphate group-containing acrylic resin, and 0.3 parts of lauryl acidphosphate. Further, 30 parts of 2-ethylhexanol, 3.3 parts of ADEKA NOLUH-814N (thickening agent produced by ADEKA Corporation, solid content:30% by mass), 0.01 parts of dimethylethanolamine (produced by KISHIDACHEMICAL Co., Ltd.), and 150 parts of ion-exchanged water, further, 20parts (resin solid content) of “Cymel 701” manufactured by Allnex Japancorporation as the melamine resin (D) were added. Next, 0.5% (solidcontent=only an effective amount of catalyst) of “CYCAT (registeredtrademark) 296-9” (weak-acidic phosphate ester, pKa (H₂O) of 1.8 ormore) manufactured by Annex Japan corporation as the weak acid catalyst(E), based on the total solid content of the acrylic emulsion of theaqueous resin (A2) and the melamine resin (D), was added while stirring,and then, 0.5 part of N,N-dimethylaminoethanol (neutralizing agent) wasadded and stirred, to obtain an aqueous base coating composition. PWC ofthe obtained coating composition was 12.0% by mass.

“Cymel 701” used in this example was an imino-methylol type melamineresin, in which the average imino group amount per one melamine nucleuswas 1.0 or more and less than 1.5, and the average methylol group amountwas 0.5 or more and less than 1.0.

Multilayer Coating Film Formation

Powernics 150 (trade name, cationic electrodeposition coating materialproduced by Nippon Paint Automotive Coatings Co., Ltd.) waselectrodeposition coated on dull steel sheet treated with zinc phosphatesuch that the thickness of the dry coating film was 20 μm, followed byheat-curing at 160° C. for 30 minutes and subsequent cooling, and thus asteel substrate was prepared as an object to be coated.

The aqueous intermediate coating composition was applied to theresulting substrate (an object to be coated) by using a rotaryatomization type electrostatic applicator such that the thickness of thedry coating film was 25 μm, and then the aqueous base coating materialwas applied by using a rotary atomization type electrostatic applicatorsuch that the thickness of the dry coating film was 15 μm, followed bypreheating at 80° C. for 3 minutes. The aqueous base coating compositionwas applied after an interval of 6 minutes from the application of theaqueous intermediate coating composition. In addition, Polyure Excel0-1200 (trade name, produced by Nippon Paint Automotive Coatings Co.,Ltd., polyisocyanate compound-containing two-components acrylicurethane-based organic solvent type clear coating material) was appliedto the coated plate by using a rotary atomization type electrostaticapplicator such that the thickness of the dry coating film was 35 μm,and then was heated and cured at 80° C. for 20 minutes, and thus aspecimen on which a multilayer coating film had been farmed wasobtained.

Example 2

Aqueous base coating composition was produced in the same manner as inExample 1 except that “Cymel 202” manufactured by Allnex Japancorporation as the melamine resin (D) was used.

“Cymel 202” used in this example was an imino-methylol type melamineresin, in which the average imino group amount per one melamine nucleuswas 1.5 or more, and the average methylol group amount was 0.5 or moreand less than 1.0.

A multilayer coating film was formed in the same manner as in Example 1except that the aqueous base coating composition obtained was used.

The amount of each component shown in the following tables is indicatedby its solid content.

Example 3

Aqueous base coating composition was produced in the same manner as inExample 1 except that 20 parts of a high imino-methylol type melamineresin (resin solid amounts, the average imino group amount per onemelamine nucleus was 2.5 or more, and the average methylol group amountwas about 1.0.) as the melamine resin (D) was used.

A multilayer coating film was formed in the same manner as in Example 1except that the aqueous base coating composition obtained was used.

Examples 4-8

Aqueous base coating compositions were produced in the same manner as inExample 3 except that the types and used amounts of the respectivecomponents were changed as shown in the following tables in theproduction of the aqueous base coating compositions.

A multilayer coating film was formed in the same manner as in Example 1except that the aqueous base coating composition obtained was used.

Examples 9-10

Aqueous base coating compositions were produced in the same manner as inExample 1 except that the types of the hydrophilicized-modifiedcarbodiimide compound (C) were changed as shown in the following tablesin the production of the aqueous base coating compositions.

A multilayer coating film was formed in the same manner as in Example 1except that the aqueous base coating composition obtained was used.

Example 11

WB-3110CB (trade name, manufactured by Nippon Paint Automotive CoatingsCo., Ltd., a conductive coating composition containing non-chlorinatedpolyolefin) was applied to a resin member (polypropylene) as an aqueousprimer for adhesion by a rotary atomization electrostatic coatingmachine, so as to have a film thickness of 15 μm. A multilayer coatingfilm was formed in the same manner as in Example 1 except that theobtained resin member was used as a coating object.

Comparative Example 1

An aqueous intermediate coating composition was prepared in the samemanner as in Example 1 except that no hydrophilicized-modifiedcarbodiimide compound (C) was used and the amounts of the respectivecomponents were changed to the amounts shown in the following table inthe production of the aqueous intermediate coating composition.

A multilayer coating film was formed in the same manner as in Example 1except that the aqueous intermediate coating composition obtained wasused.

Comparative Examples 2

Aqueous base coating composition was produced in the same manner as inExample 1 except that “Cymel 327” manufactured by Allnex JapanCorporation as the melamine resin (D) was used.

A multilayer coating film was formed in the same manner as in Example 1except that the aqueous base coating composition obtained was used.

“Cymel 327” used in this comparative example was an imino type melamineresin, in which the average imino group amount per one melamine nucleuswas 1.0 or more and less than 1.5, and the average methylol group amountwas less than 0.5.

Comparative Examples 3

Aqueous base coating composition was produced in the same manner as inExample 1 except that “Cymel 211” manufactured by Annex JapanCorporation as the melamine resin (D) was used.

A multilayer coating film was formed in the same manner as in Example 1except that the aqueous base coating composition obtained was used.

“Cymel 211” used in this comparative example was an imino type melamineresin, in which the average imino group amount per one melamine nucleuswas 1.5 or more, and the average methylol group amount was less than0.5.

Comparative example 4

Aqueous base coating composition was produced in the same manner as inExample 1 except that “Cymel 303” manufactured by Annex Japancorporation as the melamine resin (D) was used.

A multilayer coating film was formed in the same manner as in Example 1except that the aqueous base coating composition obtained was used.

“Cymel 303” used in this example was an methylol type melamine resin, inwhich the average imino group amount per one melamine nucleus was lessthan 1.0, and the average methylol group amount was less than 0.5.

Comparative Examples 5 to 7

Aqueous base coating compositions were produced in the same manner as inExample 1 except that used amounts of the respective components werechanged as shown in the following tables in the production of theaqueous base coating compositions.

A multilayer coating film was formed in the same manner as in Example 1except that the aqueous base coating composition obtained was used.

Comparative example 8

Aqueous base coating composition was produced in the same manner as inExample 1 except that the types of the aqueous polyurethane resin (F)was changed as shown in the following tables in the production of theaqueous base coating compositions.

A multilayer coating film was foamed in the same manner as in Example 1except that the aqueous base coating composition obtained was used.

The measurement of a number-average molecular weight in the examples isa value measured under the following GPC system measurement conditions.

-   Instrument: HLC-8220 GPC manufactured by Tosoh Corporation-   Column: Shodex KF-606 M, KF-603-   Flow rate: 0.6 ml/min-   Detector: RI, UV 254 nm-   Mobile phase: tetrahydrofuran-   Standard samples: TSK STANDARD POLYSTYRENE (produced by Tosoh    Corporation), A-500, A-2500, F-1, F-4, F-20, F-80, F-700,    1-phenylhexane (produced by Aldrich)

The elongation at break of each of the aqueous polyurethane resins usedin the examples and the comparative examples was measured by thefollowing procedure.

Measurement of Elongation at Break of Aqueous Polyurethane Resin

Ninety-five parts (resin solid content amount) of an aqueouspolyurethane resin and 5 parts by mass (resin solid content amount) ofthe hydrophilicized-modified carbodiimide compound (C) described inProduction Example 5 were mixed such that the two resin solid contentsthereof was 100 parts by mass in total. In a clean environment wheredusts or the like do not adhere, the mixed liquid prepared was appliedonto a flat polypropylene plate uniformly with a doctor blade such thatthe thickness of the dry coating film was 20 μm. After leaving at restat 20° C. for 10 minutes, the resulting plate was preheated at 80° C.for 3 minutes, thereby volatilizing water, and then was baked at 120° C.for 30 minutes, and thus a cured film was prepared. The cured filmobtained was subjected to a tensile performance test at a testingtemperature of −20° C. in accordance with JIS K7127 and an elongationratio at the time of breaking (elongation at break) was measured.Measurement was performed 20 times and the average of 18 measurementsexcept the maximum and the minimum values was taken as the elongation atbreak of the sample.

The multilayer coating films obtained in the above examples andcomparative examples were subjected to the following evaluations. Thetest results obtained are summarized in the following tables.

Evaluation of Water-Resistant Adhesion

The test plates obtained were immersed in warm water at 40° C. for 240hours and removed therefrom, and were then dried at 20° C. for 24 hours.Lattice-like cuts were made in the multilayer coating film on each ofthe test plates with a knife to reach the base material, so that 100crosscuts having a size of 2 mm×2 mm were made. Subsequently, anAdhesive Cellophane Tape (trademark) was affixed to each of the testplates, the tape was abruptly peeled off at 20° C., and the number ofremaining crosscut coating films was counted.

The relative merits of the coating film can be judged from the number ofpeeled crosscut sections. Even if only one crosscut section peeled, thesample is judged to be difficult to be used practically.

Evaluation of Moisture-Resistant Shrinkage

The test plates were exposed to an atmosphere with a temperature of 50°C. and a humidity of 99% for 240 hours, and then dried at 20° C. for 24hours. The state of the coating film in each of the test plates wasvisually examined, and change in appearance before and after the testwas observed. Under the following criteria, test plates with rating ◯ or◯Δ is judged to be practicality usable.

-   ◯: Almost no difference is observed in gloss and smoothness.-   ◯Δ: Slight change is observed in gloss or smoothness.-   Δ: Change is observed in gloss and smoothness.-   Δx: Change is observed in both gloss and smoothness, and especially,    change in gloss is remarkable.-   x: Remarkable difference is observed in both gloss and smoothness.

Evaluation of Moisture Blister Resistance

The test plates were exposed to an atmosphere with a temperature of 50°C. and a humidity of 99% for 240 hours, and then dried at 20° C. for 24hours. The state of the coating film in each of the test plates wasvisually examined, and change in appearance before and after the testwas observed. Under the following criteria, test plates with rating ◯ or◯Δ is judged to be practicality usable.

-   ◯: There is almost no blister.-   ◯Δ: There is a blister as small as 0.01 mm or less, which disappears    almost completely when being additionally dried at 20° C. for 24    hours.-   Δ:here is a blister as small as 0.01 mm or less, which does not    disappears even when being additionally dried at 20° C. for 24    hours.-   Δx: There is a blister as large as 0.01 mm or more and 0.05 mm or    less, which does not disappear even when being additionally dried at    20° C. for 24 hours.-   x: There is a blister as large as 0.05 mm or more, which does not    disappear even when being additionally dried at 20° C. for 24 hours.

Evaluation of Moisture-Resistant Adhesion

After exposing each of the test plates to an atmosphere having atemperature of 50° C. and a humidity of 99% for 240 hours, it was driedat 20° C. for 24 hours, and lattice-like cuts were made in themultilayer coating film of the test plate with a knife to reach the basematerial, so that 100 crosscuts having a size of 2 mm×2 mm were made.Subsequently, an Adhesive Cellophane Tape was affixed to each of thetest plates, the tape was abruptly peeled off at 20° C., and the numberof remaining crosscut coating films was counted.

The relative merits of the coating film can be judged from the number ofpeeled crosscut sections. Even if only one crosscut section peeled, thesample is judged to be difficult to be used practically.

Evaluation of Chipping Resistance

The test plates each with a layered coating film obtained in theexamples and the comparative examples were subjected to a stepping stonetest under the conditions shown below using a Gravelometer ESS-1(produced by Suga Test Instruments Co., Ltd.).

<Test Conditions>

-   Stone size: 6 to 8 mm-   Amount of stone: 0.7 to 0.8 g/piece-   Distance: 35 cm-   Shot pressure: 0.6 kg/cm²-   Shot angle: 45°-   Test temperature: −20° C.

The test plates after the stepping stone test were evaluated visuallyaccording to the following criteria. Under the following criteria, whenthe score is 4 or more, the test plate can be used practically and isjudged to be acceptable.

-   5: Almost no exfoliation is observed.-   4: There is a small exfoliated area, but almost no exfoliation is    observed at the interface between the electrodeposition coating film    and the intermediate coating film.-   3: The exfoliated area is slightly large and exfoliation is observed    at the interface between the electrodeposition coating film and the    intermediate coating film.-   2: The exfoliated area is large and exfoliation is observed at the    interface between the electrodeposition coating film and the    intemediate coating film.-   1: The exfoliated area is large and the electrodeposition coating    film is broken.

TABLE 1 Example Example Example Example Example Example 1 2 3 4 5 6Primer — — — — — — Intermediate Polyisocyanate Type EO type EO type EOtype EO type EO type EO type coating compound (B) Used 40.9 40.9 40.940.9 40.9 40.9 composition amount Hydrophilicized-modified TypeProduction Production Production Production Production Productioncarbodiimide compound example 3 example 3 example 3 example 3 example 3example 3 (C) Used 3.3 3.3 3.3 3.3 3.3 3.3 amount Aqueous resin (A1)Type AcEm-1 AcEm-1 AcEm-1 AcEm-1 AcEm-1 AcEm-1 Used 47.4 47.4 47.4 47.447.4 47.4 amount Type PE-DP PE-DP PE-DP PE-DP PE-DP PE-DP Used 8.4 8.48.4 8.4 8.4 8.4 amount Total of the above resin solid contents 100.0100.0 100.0 100.0 100.0 100.0 Aqueous base Melamine resin (D) Type C701C202 High-MF High-MF High-MF High-MF coating Used 20 20 20 20 15 25composition amount Aqueous resin (A2) Type AcEm-1 AcEm-1 AcEm-1 AcEm-1AcEm-1 AcEm-1 Used 35 35 35 35 40 40 amount Ratio (A2)/(D) 1.8 1.8 1.81.8 2.7 1.6 Weak acid catalyst (E) Type C-296-9 C-296-9 C-296-9 C-296-9C-296-9 C-296-9 (weak acid) (weak acid) (weak acid) (weak acid) (weakacid) (weak acid) Used 0.5 0.5 0.5 0.25 0.5 0.5 amount* Aqueous resinhaving a Used 25 25 25 25 25 15 hydroxyl value of less than 80 mg amountKOH/g Aqueous polyurethane Type D D D D D D resin (F) Used 20 20 20 2020 20 amount Tg −60 −60 −60 −60 −60 −60 Elongation 610 610 610 610 610610 at break Total of the above resin solid 100 100 100 100 100 100contents Clear coating composition ◯ ◯ ◯ ◯ ◯ ◯ PerformanceWater-resistant adhesion (Number of 0 0 0 0 0 0 after curing100-crosscuts peeled sections) of 80° C. for Moisture-resistantshrinkage ◯ ◯ ◯ ◯Δ ◯Δ ◯ 20 min. Moisture blister resistance ◯ ◯ ◯ ◯Δ ◯ ◯Moisture-resistant adhesion (Number 0 0 0 0 0 0 Chipping resistance (5:Good; 4 or 5 5 5 5 5 5 more: Acceptable)

TABLE 2 Example Example Example Example Example 7 8 9 10 11 Primer — — —— With primer Intermediate Polyisocyanate Type EO type EO type EO typeEO type EO type coating compound (B) Used 40.9 40.9 40.9 40.9 40.9composition amount Hydrophilicized-modified Type Production ProductionProduction Production Production carbodiimide compound example 3 example3 example 4 example 5 example 3 (C) Used 3.3 3.3 3.3 3.3 3.3 amountAqueous resin (A1) Type AcEm-1 AcEm-1 AcEm-1 AcEm-1 AcEm-1 Used 47.447.4 47.4 47.4 47.4 amount Type PE-DP PE-DP PE-DP PE-DP PE-DP Used 8.48.4 8.4 8.4 8.4 amount Total of the above resin solid contents 100.0100.0 100.0 100.0 100.0 Aqueous base Melamine resin (D) Type High-MFHigh-MF C701 C701 C701 coating Used 40 20 20 20 20 composition amountAqueous resin (A2) Type AcEm-1 AcEm-1 AcEm-1 AcEm-1 AcEm-1 Used 30 35 3535 35 amount Ratio (A2)/(D) 0.8 1.8 1.8 1.8 1.8 Weak acid catalyst (E)Type C-296-9 C-296-9 C-296-9 C-296-9 C-296-9 (weak acid) (weak acid)(weak acid) (weak acid) (weak acid) Used 0.5 0.5 0.5 0.5 0.5 amount*Aqueous resin having a Used 10 35 25 25 25 hydroxyl value of less than80 mg amount KOH/g Aqueous polyurethane Type D D D D D resin (F) Used 2010 20 20 20 amount Tg −60 −60 −60 −60 −60 Elongation 610 610 610 610 610at break Total of the above resin solid 100 100 100 100 100 contentsClear coating composition ◯ ◯ ◯ ◯ ◯ Performance Water-resistant adhesion(Number of 0 0 0 0 0 after curing 100-crosscuts peeled sections) of 80°C. for Moisture-resistant shrinkage ◯Δ ◯ ◯ ◯ ◯ 20 min. Moisture blisterresistance ◯Δ ◯ ◯ ◯ ◯ Moisture-resistant adhesion (Number 0 0 0 0 0 of100-crosscuts peeled sections) Chipping resistance (5: Good; 4 or 5 4 55 5 more: Acceptable)

TABLE 3 Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar-ative ative ative ative ative ative ative ative example example exampleexample example example example example 1 2 3 4 5 6 7 8 Primer — — — — —— — — Intermediate Polyisocyanate Type EO type EO type EO type EO typeEO type EO type EO type EO type coating compound (B) Used 40.9 40.9 40.940.9 40.9 40.9 40.9 40.9 composition amount Hydrophilicized-modifiedType Production Production Production Production Production ProductionProduction Production carbodiimide compound example 3 example 3 example3 example 3 example 3 example 3 example 3 example 3 (C) Used 0 3.3 3.33.3 3.3 3.3 3.3 3.3 amount Aqueous resin (A1) Type AcEm-1 AcEm-1 AcEm-1AcEm-1 AcEm-1 AcEm-1 AcEm-1 AcEm-1 Used 50.7 47.4 47.4 47.4 47.4 47.447.4 47.4 amount Type PE-DP PE-DP PE-DP PE-DP PE-DP PE-DP PE-DP PE-DPUsed 8.4 8.4 8.4 8.4 8.4 8.4 8.4 8.4 amount Total of the above resinsolid contents 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 AqueousMelamine resin (D) Type C701 C327 C211 C303 C701 C701 C701 C701 basecoating Used 20 20 20 20 20 20 5 20 composition amount Aqueous resin(A2) Type AcEm-1 AcEm-1 AcEm-1 AcEm-1 AcEm-1 AcEm-1 AcEm-1 AcEm-1 Used35 35 35 35 35 10 50 35 amount Ratio (A2)/(D) 1.8 1.8 1.8 1.8 1.8 0.510.0 1.8 Weak acid catalyst (E) Type C-296-9 C-296-9 C-296-9 C-296-9C-296-9 C-296-9 C-296-9 C-296-9 (weak (weak (weak (weak (weak (weak(weak (weak acid) acid) acid) acid) acid) acid) acid) acid) Used 0.5 0.50.5 0.5 0 0.5 0.5 0.5 amount* Aqueous resin having a Used 25 25 25 25 2555 25 25 hydroxyl value of less amount than 80 mg KOH/g Aqueouspolyurethane Type D D D D D D D A resin (F) Used 20 20 20 20 20 20 20 20amount Tg −60 −60 −60 −60 −60 −60 −60 −10 Elongation 610 610 610 610 610610 610 12 at break Total of the above resin solid 100 100 100 100 100105 100 100 Clear coating composition ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ PerformanceWater-resistant adhesion (Number of 10 0 0 0 0 0 0 0 after curing100-crosscuts peeled sections) of 80° C. for Moisture-resistantshrinkage ◯ Δ Δ X X Δ ΔX ◯Δ 20 min. Moisture blister resistance ◯ ΔX Δ XX Δ Δ Δ Moisture-resistant adhesion (Number 30 0 0 0 0 0 0 0 Chippingresistance (5: Good: 4 or 5 5 5 2 2 3 3 3 more: Acceptable)

The types of the aqueous polyurethane resins (F) shown in the abovetables are as follows.

-   A: N9603 (produced by Kusumoto Chemicals, Ltd.), solid    concentration:-   34%, Tg: −10° C., elongation at break: 12%-   D: PERMARIN U150 (produced by Sanyo Chemical Industries, Ltd.),    solid concentration: 30%, Tg: −60° C., elongation at break: 610%

In the above tables, “AcEm-1” shown in the columns of aqueous resinrepresents the acrylic emulsion obtained in Production Example 1.“PE-DP” represents the aqueous polyester dispersion having a hydroxylgroup and a carboxyl group obtained in Production Example 2.

“C-” shown in the columns of melamine resin (D) is an abbreviation for“Cymel”. “High-MF” represents a high imino-methylol type melamine resin,having adjusted average imino group amount of 2.5 or more, and averagemethylol group amount of about 1.0, per one melamine nucleus.

“C-” shown in the field of weak acid catalyst (E) is an abbreviation for“CYCAT”.

The amount * of the weak acid catalyst (E) is a % by mass based on thesolid content mass of the aqueous resin (A2) and the melamine resin (D)contained in the aqueous base coating composition, and is a valuecalculated by the following formula.

Amount of weak acid catalyst=(E)/((A2)+(D)) (% by mass)

Each of the multilayer coating films obtained in the examples wasconfirmed to have excellent water resistance, moisture resistance, andchipping resistance even after the multilayer coating films weresubjected to baking and curing at a low-temperature condition of 80° C.

On the other hand, the multilayer coating films obtained in thecomparative examples were confirmed to be inferior in one or two or moreof water resistance, moisture resistance, and chipping resistance.

Comparative Example 1 is an example in which no hydrophilicized-modifiedcarbodiimide compound (C) is contained in the aqueous intermediatecoating composition. In this case, the multilayer coating film obtainedis clearly inferior in water resistance.

Comparative Examples 2-4 are examples in which type of the melamineresin (D) contained in the aqueous base coating composition is out ofthe range of claims according to the present invention. In these cases,the multilayer coating film obtained is clearly inferior inmoisture-resistance.

Comparative Example 5 is an example in which no weak acid catalyst (E)is contained in the aqueous base coating composition.

In this case, the multilayer coating film obtained is inferior inchipping resistance and moisture-resistance.

Comparative Examples 6-7 are examples in which a mass ratio of theaqueous resin and the melamine resin (D) contained in the aqueous basecoating composition is out of the range of claims according to thepresent invention. In these cases, the multilayer coating film obtainedis clearly inferior in chipping resistance.

Comparative Example 8 is an example in which property of the aqueouspolyurethane resin (F) contained in the aqueous base coating compositionis out of the range of claims according to the present invention. Inthese cases, the multilayer coating film obtained is clearly inferior inchipping resistance. In addition, it is also inferior inmoisture-resistance (moisture blister resistance). Humidity resistanceevaluation test is one type of water resistance evaluation test of thecoating film.

In addition, the moisture resistance evaluation test is generally asevere evaluation test in which stricter conditions are imposed ascompared with water resistance adhesion evaluation tests. Accordingly,also in Comparative Example 8, sufficient water resistance was notobtained as compared with the examples.

According to the examples and the comparative examples described above,an advantageous effect of the present invention such that a curing(crosslinking) reaction can be performed sufficiently under curingconditions at lower temperatures than before is demonstrated by forminga multilayer coating film using a specific aqueous intermediate coatingcomposition and a specific aqueous base coating composition, especiallyby forming an uncured coating film using an aqueous intermediate coatingcomposition containing the above-described specific components, andsubsequently using an aqueous base coating composition containing anaqueous resin having a hydroxyl group and a carboxyl group (A2), amelamine resin (D), a weak acid catalyst (E) and an aqueous polyurethaneresin (F) in specific amounts.

Moreover, it can be said that the technical significance of the presentinvention is proved sufficiently when the above demonstration isconsidered in connection with the mechanism of action imparted by theconstitution of the present invention described herein.

INDUSTRIAL APPLICABILITY

The method for forming a multilayer coating film of the presentinvention is advantageous in that a curing reaction proceeds well evenunder heating conditions of low temperature conditions (e.g., heatingconditions at 100° C. or less), so that a cured coating film havingexcellent coating film properties can be obtained. The method forforming a multilayer coating film of the present invention can besuitably used, for example, for coating an object to be coated having asteel plate part and a resin part. The method for forming a multilayercoating film of the present invention is also an effective method as ameans for reducing an environmental load such as energy saving and CO₂emission reduction.

1. A method for forming a multilayer coating film, wherein the methodcomprises: an intermediate coating film formation step of applying anaqueous intermediate coating composition to an object to be coated toform an uncured intermediate coating film; a base coating film formationstep of applying an aqueous base coating composition onto a resultinguncured intermediate coating film to form an uncured base coating film;and a curing step of curing the resulting uncured intermediate coatingfilm and the base coating film by heating, wherein the aqueousintermediate coating composition is an aqueous intermediate coatingcomposition comprising: an aqueous resin having a hydroxyl group and acarboxyl group (A1); a polyisocyanate compound (B); and ahydrophilicized-modified carbodiimide compound (C), the aqueous basecoating composition is an aqueous base coating composition comprising:an aqueous resin having a hydroxyl group and a carboxyl group (A2); amelamine resin (D), a weak acid catalyst (E); and an aqueouspolyurethane resin (F), wherein the aqueous resin having a hydroxylgroup and a carboxyl group (A1) contained in the aqueous intermediatecoating composition has a hydroxyl value of 80 to 200 mgKOH/g and anacid value of 10 to 40 mgKOH/g in terms of resin solid content, theaqueous resin having a hydroxyl group and a carboxyl group (A2)contained in the aqueous base coating composition has a hydroxyl valueof 80 to 200 mgKOH/g in terms of resin solid content, thehydrophilicized-modified carbodiimide compound (C) is a compoundrepresented by a formula (I), (II), or (III) below,[Chemical Formula 1]YOCONH—X—NHCOO-—Z—OCONH—X—NHCOOY   (I) wherein each X is a bifunctionalorganic group having at least one carbodiimide group, Y is each same ordifferent structure resulting from elimination of a hydroxyl group froma polyalkylene glycol monoalkyl ether, and Z is a structure resultingfrom elimination of a hydroxyl group from a bifunctional polyol having anumber-average molecular weight of 200 to 5,000,

wherein each X is a bifunctional organic group having at least onecarbodiimide group, Y is each same or different structure resulting fromelimination of a hydroxyl group from a polyalkylene glycol monoalkylether, R⁰ is hydrogen, a methyl group or an ethyl group, each R¹ is analkylene group having 4 or less carbon atoms, n is 0 or 1, and each m isa number from 0 to 60,[Chemical Formula 3]YOCONH—X—NHCOOY   (III) wherein X is a bifunctional organic group havingat least one carbodiimide group, and Y is each same or differentstructure resulting from elimination of a hydroxyl group from apolyalkylene glycol monoalkyl ether, and the melamine resin (D) has anaverage imino group amount of 1.0 or more, and an average methylol groupamount of 0.5 or more, per one melamine nucleus, a mass ratio of theaqueous resin (A2) and the melamine resin (D) contained in the aqueousbase coating composition is in the range of (A2)/(D)=0.7 to 3 in termsof solid content, an amount of the weak acid catalyst (E) contained inthe aqueous base coating composition is within the range of 0.1 to 10.0parts by mass, based on 100 parts by mass of solid contents ((A2)+(D))of the aqueous resin (A2) and the melamine resin (D) contained in theaqueous base coating composition, the aqueous polyurethane resin (F) hasa glass transition point (Tg) of −50° C. or less, and a cured film ofthe aqueous polyurethane resin (F) has an elongation at break of 400% ormore at −20° C.
 2. The method for forming a multilayer coating filmaccording to claim 1, wherein a content of the hydrophilicized-modifiedcarbodiimide compound (C) is 1 to 8% by mass based on a resin solidcontent of the aqueous intermediate coating composition.
 3. The methodfor forming a multilayer coating film according to claim 1, wherein acontent of the aqueous polyurethane resin (F) is 8% by mass or more and30% by mass or less based on a resin solid content of the aqueous basecoating composition.
 4. The method for forming a multilayer coating filmaccording to claim 1, wherein a ratio of an equivalent of a carbodiimidegroup of the hydrophilicized-modified carbodiimide compound (C) to anequivalent of an acid group of the aqueous resin (A1) contained in theaqueous intermediate coating composition is 0.1 to 0.6.
 5. The methodfor forming a multilayer coating film according to claim 1, wherein anamount of the weak acid catalyst (E) is within the range of 0.1 to 5.0parts by mass, based on 100 parts by mass of solid contents ((A2)+(D))of the aqueous resin (A2) and the melamine resin (D) contained in theaqueous base coating composition.
 6. The method for forming a multilayercoating film according to claim 1, wherein the weak acid catalyst (E)comprises a phosphate ester compound.
 7. The method for forming amultilayer coating film according to claim 1, wherein the aqueous basecoating composition further comprises an aqueous resin (G) having ahydroxyl value of less than 80 mgKOH/g.
 8. The method for forming amultilayer coating film according to claim 1, wherein the object to becoated includes a steel plate part and a resin part.
 9. The method forforming a multilayer coating film according to claim 1, wherein themethod further comprises a clear coating film formation step of applyinga clear coating composition onto the uncured base coating film obtainedin the base coating film formation step to form an uncured clear coatingfilm, wherein the curing step is a step of curing the resulting uncuredintermediate coating film, base coating film, and clear coating film byheating.
 10. The method for forming a multilayer coating film accordingto claim 1, wherein a heating temperature in the curing step is 70 to120° C.